TheUnited States of America was formed afterthirteen British colonies in North Americadeclared independence from theBritish Empire on July 4, 1776. In theLee Resolution, passed by theSecond Continental Congress two days prior, the colonies resolved that they were free and independent states. The union was formalized in theArticles of Confederation, whichcame into force on March 1, 1781, after being ratified by all 13 states. Their independence was recognized byGreat Britain in theTreaty of Paris of 1783, which concluded theAmerican Revolutionary War. This effectively doubled the size of the colonies, now able to stretch west past theProclamation Line to theMississippi River. This land was organized into territories and then states, though there remained some conflict with the sea-to-sea grants claimed by some of the original colonies. In time, these grants wereceded to the federal government.
The first great expansion of the country came with theLouisiana Purchase of 1803, which doubled the country's territory, although the southeastern border withSpanish Florida was the subject of much dispute until it and Spanish claims to theOregon Country were ceded to the US in 1821. The Oregon Country gave the United States access to thePacific Ocean, though it was shared for a time with theUnited Kingdom.[2] Theannexation of theRepublic of Texas in 1845 led directly to theMexican–American War, after which the victorious United States obtained the northern half ofMexico's territory, including what was quickly made the state ofCalifornia.[3]
As the development of the country moved west, however, the question ofslavery became more important, with vigorous debate over whether the new territories would allow slavery and events such as theMissouri Compromise andBleeding Kansas. This came to a head in 1860 and 1861, when the governments of thesouthern states proclaimed their secession from the country and formed theConfederate States of America. TheAmerican Civil War led to the defeat of the Confederacy in 1865 and the eventual readmission of the states to theUnited States Congress. The cultural endeavor and pursuit ofmanifest destiny provided a strong impetus for westward expansion in the 19th century.
The United States began expanding beyond North America in 1856 with the passage of theGuano Islands Act, causing many small and uninhabited, but economically important, islands in theCaribbean Sea and the Pacific Ocean to be claimed.[4] Most of these claims were eventually abandoned, largely because of competing claims from other countries. The Pacific expansion culminated in theannexation of Hawaii in 1898, after theoverthrow of its government five years previously.Alaska, the last major acquisition in North America, waspurchased fromRussia in 1867. Support for theindependence of Cuba from theSpanish Empire, and the sinking of theUSSMaine, led to theSpanish–American War in 1898, in which the United States gainedPuerto Rico,Guam, and thePhilippines, and occupied Cuba for several years.American Samoa was acquired by the United States in 1900 after the end of theSecond Samoan Civil War.[5] The United Statespurchased the U.S. Virgin Islands from Denmark in 1917.[6] Puerto Rico and Guam remain territories, and the Philippines became independent in 1946, after being a major theater ofWorld War II.
Following the war,many islands wereentrusted to the U.S. by theUnited Nations,[7] and while theNorthern Mariana Islands became a U.S. territory, theMarshall Islands,Federated States of Micronesia, andPalau emerged from the trust territory as independent nations. The last major international change was the acquisition in 1904, and return toPanama in 1979, of thePanama Canal Zone, an unincorporated US territory which controlled thePanama Canal. The final cession of formal control over the region was made to Panama in 1999.
States have generally retained their initial borders once established. Only three states (Kentucky,Maine, andWest Virginia) have been created directly from area belonging to another state (although at the time of admission, Vermont agreed to a monetary payment for New York to relinquish its claim); all of the other states were created from federal territories or from acquisitions. Four states (Louisiana,Missouri,Nevada, andPennsylvania) have expanded substantially by acquiring additional federal territory after their initialadmission to the Union. In 1912,Arizona was the last state established in thecontiguous United States, commonly called the "lower 48". In 1959,Hawaii was the 50th and most recent state admitted.
| Date | Event | Change map |
|---|---|---|
| July 4, 1776 | Thirteen colonies of theKingdom of Great Britain in North America collectivelydeclared their independence as the United States of America,[a] though several colonies had already individually declared independence:[8]
The capital was not specifically established; at the time, theContinental Congress met inPhiladelphia.[25][26] Many states had only vaguely defined borders at the time of independence, with only four states having had their boundaries fully surveyed. The borders of North Carolina were particularly poorly surveyed, its border with South Carolina having been done in several pieces, none of which truly matched the spirit of the charter,[27][28] and its border with Virginia was only surveyed roughly halfway inland from the sea, and a few miles north of ideal. Several northeastern states had overlapping claims: Connecticut, Massachusetts Bay, and New York all claimed land west of their settled borders, overlapping with each other and with a sizable claim by Virginia. Of the three, only Connecticut seriously pursued its claims, while Virginia is considered to have had the most legitimate claim to the vast northwest, dividing it into counties and maintaining some limited control. The entirety of the new United States was claimed by Great Britain, includingMachias Seal Island andNorth Rock, two small islands off the northeast coast which remain disputed up to the present.[29] |  Disputes:
|
| September 20, 1776 | The Counties of New Castle, Kent, and Sussex, upon Delaware enacted aconstitution, renaming itselfthe Delaware State.[30] |  |
| September 28, 1776 | The State of Pennsylvania enacted aconstitution, renaming itself theCommonwealth of Pennsylvania.[31] | no change to map |
| December 20, 1776 | To avoidBritish forces who were advancing onPhiladelphia, theContinental Congress began meeting inBaltimore.[25][26] |  |
| January 15, 1777 | The northeastern region ofNew York, known as theNew Hampshire Grants, declared independence asNew Connecticut.[32][33][34] | Disputes:
|
| March 4, 1777 | TheContinental Congress returned toPhiladelphia after the threat to it byBritish forces ended.[25][26] |  |
| June 4, 1777 | New Connecticut was renamedVermont.[34][32] | Disputes:
|
| September 27, 1777 | TheContinental Congress fledPhiladelphia after the American defeat at theBattle of Brandywine, and briefly met inLancaster, Pennsylvania[25][26] |  |
| September 30, 1777 | TheContinental Congress continued to move away fromPhiladelphia, settling inYork, Pennsylvania.[25][26] |  |
| June 11, 1778 | Vermont claimed what was called the "East Union", consisting of some towns inNew Hampshire that petitioned on March 12, 1778, to join with Vermont, out of concern that their state was focusing too much on its coastal region. Vermont never gained full control over the area.[32][35][36][37][b] | Disputes: |
| October 21, 1778 | Under pressure from theContinental Congress,Vermont rescinded the annexation of the East Union; the legislature declared on February 12, 1779, that the East Union should be considered null from its beginning.[35][36][37] | Disputes: |
| July 2, 1779 | TheContinental Congress returned toPhiladelphia followingBritishwithdrawal.[25][26] |  |
| August 31, 1779 | Virginia surrendered itsclaim to southwestPennsylvania.[20][38] |  |
| March 23, 1780 | North Carolina andVirginia surveyed their border further inland. Virginia's survey reached theTennessee River on this date,[39] while North Carolina's team stopped at theCumberland Gap and filed their survey on November 17, 1779.[40] The two surveys were roughly two miles apart, creating a thin area claimed by both states. While the border was intended to follow36°30′ north, early surveying errors caused it to veer north of that, reaching a distance of almost ten miles off by the time it reached the Tennessee River.[41][24] |  |
| October 25, 1780 | The State of Massachusetts Bay enacted aconstitution, renaming itself theCommonwealth of Massachusetts. |  |
| March 1, 1781 | TheArticles of Confederation entered into force.[42] | no change to map |
| April 4, 1781 | Vermont again claimed an East Union, consisting of some towns inNew Hampshire that wished to join with Vermont; more towns were interested than during the first attempt in 1778, though again, the exact extent of the borders is unknown. Vermont never gained full control over the area.[32][43][36][37][c] | Disputes: |
| June 16, 1781 | Vermont claimed what was called the "West Union", consisting of some towns inNew York, mainly to counterbalance Vermont's attempt at eastward expansion. Vermont never gained full control over the area.[32][36][44][45] The specific date this occurred is unclear; sources suggest June 16, June 26, and July 18.[d] | Disputes: |
| February 22, 1782 | Vermont abandoned its attempts to annex the East Union fromNew Hampshire and the West Union fromNew York.[32][37][45][46] | Disputes: |
| October 29, 1782 | The federal government accepted thecession fromNew York of its western claims, which the state ceded on February 19, 1780, and executed on March 1, 1781; New York proclaimed its new western border to be a line drawn south from the western end ofLake Ontario. At its maximum interpretation, the state had claimed an area bounded byLake Erie,Lake Huron andLake Michigan; to theIllinois,Mississippi, andTennessee Rivers; and north along theAppalachian Mountains, ending at the border withPennsylvania.[47] It is unclear from where this claim came; many sources state that New York had surrendered it, but very few elaborate on how it was obtained. One source states that it was a cession by theSix Nations, who had conquered much of the region.[48] However, New York never seriously enforced these claims. The cession included the small tip of New York north of Pennsylvania, which came to be known as theErie Triangle.[49][18] |  |
| December 30, 1782 | TheCongress of the Confederation declared that the land thatConnecticut claimed in northernPennsylvania, a westward extension of Connecticut's borders, was part of Pennsylvania, thus attempting to end thePennamite–Yankee War.[50][20] While conflict would continue for some time, this was the end of the formal claim by Connecticut. |  |
| June 30, 1783 | ThePennsylvania Mutiny of 1783, and thePennsylvania government reaction to it, caused theCongress of the Confederation to leavePhiladelphia forPrinceton.[25] |  |
| November 26, 1783 | TheCongress of the Confederation reconvened inAnnapolis.[25] |  |
| March 1, 1784 | Virginia cededits claims northwest of theOhio River to the federal government.[51][24]Connecticut continued to claim its western lands that had overlapped with Virginia's cession. |  |
| May 12, 1784 | Great Britainrecognized the independence of the United States, ending its claim to the country.[52][53][e] The treaty ended theAmerican Revolutionary War, though military action had largely ended after theFranco-American victory atYorktown on October 19, 1781. Because of ambiguities and poor knowledge of geography, the treaty was unclear in several areas:
ThePeace of Paris also involved treaties withFrance andSpain, with Great Britain ceding theFloridas toSpain. During their ownership ofWest Florida, the British had moved its border north, and the cession to Spain appeared to apply to the full extent of the British colony. However, the British-American treaty granted the extension of West Florida to the United States, where it presumably enlargedGeorgia south to31° north, indicating that only the original definition of West Florida was to be ceded to Spain. The local Spanish governors also made a move to occupy forts along theMississippi River, with claims to everything south of theTennessee River; it is unknown how official or strong these claims were, and they are not mapped as they are in conflict with the other Spanish claim involving the border of West Florida.[54] |  Disputes:  |
| Date | Event | Change map |
|---|---|---|
| August 23, 1784 | A region in centralNorth Carolina (modern-day easternTennessee), unhappy with the state's governance over the area, declared independence from the state as theState of Franklin.[f][56][57] The government of Franklin held some control over the area, and petitioned for statehood, receiving support from seven of the nine states required, but would only last a few years.[58][59] | Unofficial change: |
| November 1, 1784 | TheCongress of the Confederation moved for a short time toTrenton.[25] |  |
| January 11, 1785 | TheCongress of the Confederation moved toNew York, and would settle there for five years.[25] |  |
| April 19, 1785 | The federal government accepted thecession fromMassachusetts of its extreme western claim, which was never seriously enforced.[g][47][14] | Change on paper only: |
| August 23, 1785 | The survey of thePennsylvania–Virginia border was completed.[20][38] | |
| September 13, 1786 | Connecticut surrendered its western claim to the federal government except for itsWestern Reserve, though it is unclear how much control they held over the ceded region.[h][9][60] | Change on paper only: |
| December 16, 1786 | Massachusettssurrendered its claim to westernNew York, though it is unclear if Massachusetts ever held control over the region, as the claim was to the "soil, not the sovereignty".[i][47][14] This land was later known as thePhelps and Gorham Purchase. | Change on paper only: |
| July 13, 1787 | The Territory Northwest of theRiver Ohio, ceded earlier byVirginia, wasorganized and commonly became known as theNorthwest Territory.[61][62] |  |
| July 21, 1787 | The survey of theMassachusetts–New York border was completed.[14][63] | |
| August 9, 1787 | South Carolina ceded its western claim to the federal government,[64][23] though it was a result of inaccurate geography and South Carolina never actually held claim to this land. The claim was of a strip of land between the border ofNorth Carolina and the source of theTugaloo River but, unknown at the time, the river originated in North Carolina. The eastern part of this cession would be given toGeorgia in 1802, despite Georgia technically already having claim to the land.[65] | Change on paper only: |
| December 7, 1787 | Delaware became the first state to ratify theUnited States Constitution.[66] | no change to map |
| December 12, 1787 | Pennsylvania became the second state to ratify theConstitution.[67] | |
| December 18, 1787 | New Jersey became the third state to ratify theConstitution.[68] | |
| January 2, 1788 | Georgia became the fourth state to ratify theConstitution.[69] | |
| January 9, 1788 | Connecticut became the fifth state to ratify theConstitution.[70] | |
| February 6, 1788 | Massachusetts became the sixth state to ratify theConstitution.[71] | |
| April 28, 1788 | Maryland became the seventh state to ratify theConstitution.[72] | |
| May 23, 1788 | South Carolina became the eighth state to ratify theConstitution.[73] | |
| June 21, 1788 | New Hampshire became the ninth state to ratify theConstitution;[74] at this point, the Constitution became the active governing document of those nine states having ratified the same (perArticle VII). | |
| June 25, 1788 | Virginia became the tenth state to ratify theConstitution.[75] | |
| July 26, 1788 | New York became the eleventh state to ratify theConstitution.[76] | |
| February 1789 | John Sevier, governor of theState of Franklin, pledged allegiance toNorth Carolina at the February term of theGreene County Court, effectively ending the claimed independence of Franklin.[58][77] | Unofficial change: |
| August 7, 1789 | TheNorthwest Territory was reorganized under theConstitution.[78] | no change to map |
| September 29, 1789 | The survey of theNew York–Pennsylvania border, completed in 1787, was approved by Pennsylvania on this date;[79] it would not be approved by New York until 1829.[18] | |
| November 21, 1789 | North Carolina became the twelfth state to ratify theConstitution.[80] | no change to map |
| April 2, 1790 | North Carolina ceded its western half to the federal government.[j][81][59] |  |
| May 26, 1790 | The land recently ceded byNorth Carolina was organized as the Territory South of the River Ohio, commonly known as theSouthwest Territory.[59][82] |  |
| May 29, 1790 | Rhode Island became the thirteenth state to ratify theConstitution.[83] | no change to map |
| October 10, 1790 | The survey of theNew York–Erie Triangle border was completed.[18][84][85] | |
| December 6, 1790 | Per theResidence Act, theCongress of the United States relocated toPhiladelphia for ten years until afederal district was built and ready.[25][26][86] |  |
| March 4, 1791 | Vermont, which had been considered part ofNew York despiteacting independently since 1777, was admitted as the fourteenth state.[k][32][87] |  Disputes:  |
| March 30, 1791 | Afederal district planned to house the federal government by 1800 wasformed from land ceded byMaryland andVirginia,[88][89] consisting of a 100 square mile diamond, with its southern tip atJones Point, straddling thePotomac River. It did not yet have a formal name, being simply referred to as the federal district; in September 1791, the commissioners in charge of planning the city would term it the "Territory of Columbia", and various laws refer to a District of Columbia. The area does not appear to have been formally named "District of Columbia" until theorganic act of 1871.[90] |  |
| March 3, 1792 | Pennsylvania purchased theErie Triangle from the federal government.[20] |  |
| June 1, 1792 | The western half ofVirginia, which the state had agreed in 1789 to cede to the federal government,[91] was admitted as the fifteenth state, the Commonwealth ofKentucky.[l][93][92] |  |
| June 12, 1792 | The Delaware State enacted anew constitution, with the reorganized government being known as theState of Delaware.[94] | no change to map |
| January 1, 1793 | The survey of thefederal district's borders was completed.[89][95] | |
| August 3, 1795 | Representatives of the United States and theWestern Confederacy sign theTreaty of Greenville, ending theNorthwest Indian War and ceding most of the modern state ofOhio to United States control.[96] | no change to map |
| February 29, 1796 | Great Britainagreed to abandon several forts in the northwest that it still occupied, includingDetroit. The Jay Treaty also provided for commissions to determine the border between thenorthwestern point of the Lake of the Woods and thesource of theMississippi River, and which river to consider theSt. Croix River.[97] | |
| April 25, 1796 | The northern half ofWest Florida wasceded bySpain, resolving the dispute over the region.[98][99] | Disputes: |
| June 1, 1796 | TheSouthwest Territory was admitted as the sixteenth state,Tennessee.[59][100] |  |
| April 7, 1798 | In response to theYazoo Land Fraud, an act authorized PresidentJohn Adams to appoint commissioners to negotiate withGeorgia about ceding its western land. The act createdMississippi Territory from the southwestern quarter of Georgia in the region recently ceded byWest Florida, while maintaining that Georgia still held rights over the territory.[101][102] |  |
| October 25, 1798 | Commissioners agreed on the source of theSt. Croix River, setting the lower portion of the border betweenMassachusetts andGreat Britain and, thus, where the eastern north–south line originated.[103] |  Disputes:  |
| October 15, 1799 | The first half of the survey of theNorth Carolina–Tennessee border was completed.[19][104] | |
| January 13, 1800 | The survey of theKentucky–Virginia border was completed.[24][105] | |
| May 18, 1800 | This is the closest date for the submission of the survey of the southern border ofMississippi Territory along 31° north,[12] as this is the day surveyorAndrew Ellicott arrived back inPhiladelphia after completing the survey.[106] | |
| June 9, 1800 | Connecticut ceded itsWestern Reserve to the federal government;[107] it presumably was then considered part of theNorthwest Territory. The act doing so was passed in Congress on April 28, 1800, and Connecticut approved it on this date.[108] |  |
| July 4, 1800 | Indiana Territory was organized from the western half ofNorthwest Territory.[m][110][109] |  |
| November 17, 1800 | TheCongress of the United States moved toWashington, now built and ready to be the capital.[25] This was two weeks before the December 1 date established in theResidence Act; PresidentJohn Adams urged Congress to move early in hopes of securing enough Southern votes to be re-elected, though this failed.[111] |  |
| January 1, 1801 | TheKingdom of Great Britainunited with theKingdom of Ireland, with the reorganized government being known as theUnited Kingdom.[112] | Disputes: |
| February 27, 1801 | The federal district wasorganized as theDistrict of Columbia.[89][113] | no change to map |
| April 26, 1802 | Georgia ceded its western half, known as theYazoo Lands, to the federal government.[n] At the same time, the federal government ceded to Georgia the eastern portion of the land previously ceded bySouth Carolina, though in reality Georgia technically already held title to the land, as the description of the earlier cession was based on an erroneous understanding of geography.[12] |  |
| March 1, 1803 | The southern half of theNorthwest Territory, along with a thin sliver ofIndiana Territory, was admitted as the seventeenth state,Ohio. The remainder of the Northwest Territory was transferred to Indiana Territory.[114][61] The western border was a line due north from the mouth of theGreat Miami River; the federal definition of the northern border was a line drawn east from the southern tip ofLake Michigan, whereas the Ohio Constitution stated the line should run from the southern tip of Lake Michigan to the most northerly cape ofMaumee Bay, essentially the western tip ofLake Erie. The confusion caused by these varying descriptions of the state's borders, combined with inaccurate knowledge of geography, as no one at the time knew just how far south Lake Michigan extended, would lead to theconflict over the Toledo Strip. |  |
| November 3, 1803 | The border betweenTennessee andVirginia was resurveyed and established, ending the dispute over that part of the border. The border betweenKentucky andTennessee, despite following the original survey, remained vaguely defined.[41][115] |  |
| November 30, 1803 | The "Southwick Jog" was transferred fromConnecticut toMassachusetts, to put to rest long-standing disagreements over the border between the two states.[14] The final report of the commissioners was delivered this day;[116] it is unclear when the change formally occurred. |  |
| Date | Event | Change map |
|---|---|---|
| December 20, 1803 | The United StatespurchasedLouisiana fromFrance. This is the date of the formal turnover inNew Orleans; the purchase was completed on April 30, 1803.[117] The transfer would be recognized inSt. Louis inUpper Louisiana on March 10, 1804, known asThree Flags Day. The acquisition expanded the United States to the whole of theMississippi River basin,[o] but the extent of what constituted Louisiana in the south was disputed withSpain: the United States claimed the purchase included the part ofWest Florida west of thePerdido River, whereas Spain claimed it ended at the western border of West Florida;[p][118] and the southwestern border withNew Spain wasdisputed, as the United States claimed theSabine River as the border, but Spain maintained it was theCalcasieu River and others.[117] |  Disputes:
|
| March 27, 1804 | The land betweenTennessee andMississippi Territory previously ceded byGeorgia was assigned to Mississippi Territory.[102][119] |  |
| October 1, 1804 | Orleans Territory was organized from theLouisiana Purchase south of33° north, with the remainder being designated theDistrict of Louisiana and placed under the jurisdiction ofIndiana Territory.[120][121] |  |
| June 30, 1805 | Michigan Territory was organized fromIndiana Territory, north of a line east from the southern tip ofLake Michigan, and east of a line north from the lake's northern tip.[122][123] The southeastern portion of the border technically conflicted with the definition ofOhio, which claimed theToledo Strip north of that line; however, the exact position of Lake Michigan was not yet known. |  |
| July 4, 1805 | TheDistrict of Louisiana was organized asLouisiana Territory.[121][124] |  |
| 1806 | The survey of the northernOrleans Territory border between theMississippi River and theRed River was completed by 1806.[121][q] | |
| March 1, 1809 | Illinois Territory was organized from the western half ofIndiana Territory.[r][127][126] |  |
| September 26, 1810 | TheRepublic of West Florida declared independence fromSpain, claiming the area ofWest Florida west of thePerdido River. It maintained some control over its territory.[128] | Disputes: |
| December 10, 1810 | Armed forces led byWilliam C. C. Claiborne took possession of the portion ofWest Florida west of thePearl River, following a proclamation on October 27, 1810, by PresidentJames Madison to do so. The United States had considered the region part of theLouisiana Purchase, including the area which had revolted againstSpanish Florida and formed theRepublic of West Florida. Madison's proclamation stated that it was to be "taken as part" ofOrleans Territory.[118][117][129] The land west of Mobile Bay to the Pearl River was occupied and annexedde facto by the military in 1811.[130]: 2a (mapArchived 2016-09-16 at theWayback Machine) | Disputes: |
| April 30, 1812 | Most ofOrleans Territory was admitted as the eighteenth state,Louisiana.[s][121][131] The southeastern remainder presumably became unorganized territory, as it had no definition for a short time. |  |
| May 14, 1812 | The claimed portion ofWest Florida east of thePearl River was assigned toMississippi Territory, though the area around Mobile Bay remained under the control ofSpanish Florida.[102][132] The United States militarily occupied Mobile and the surrounding area up to thePerdido River in April 1813. |  |
| June 4, 1812 | Since its name was now shared with the state ofLouisiana,Louisiana Territory was renamedMissouri Territory.[133][134] |  |
| August 4, 1812 | The remaining claimed portion ofWest Florida, west of thePearl River, was added toLouisiana, following the assent of that state to an act passed by Congress on April 14, 1812.[135][136] |  |
| August 16, 1812 | During theWar of 1812, the garrison atFort Detroitsurrendered, leading theUnited Kingdom to occupyDetroit, the capital and population center ofMichigan Territory.[137] | Disputes: |
| September 29, 1813 | The British withdrew fromFort Detroit following theBattle of Lake Erie, allowing American forces to regain control overMichigan Territory.[137][138] | Disputes: |
| August 24, 1814 | British forcescapture and burn Washington, but are forced to withdraw the next day. The functions of the capital were only momentarily suspended, though PresidentJames Madison took refuge inBrookville, Maryland.[139] | no change to map |
| October 25, 1814 | The survey of theNew York–Vermont border was completed.[32][140] | |
| October 25, 1815 | The final part of the survey ofNorth Carolina–South Carolina border was completed.[19][141] | |
| December 11, 1816 | The southern part ofIndiana Territory, along with small parts ofIllinois Territory andMichigan Territory, were admitted as the nineteenth state,Indiana.[t] The remainder of Indiana Territory acrossLake Michigan became unorganized territory.[109][142] |  |
| March 3, 1817 | Alabama Territory was organized from the eastern half ofMississippi Territory.[u][144][143] |  |
| December 10, 1817 | Mississippi Territory was admitted as the twentieth state,Mississippi.[102][145] |  |
| January 29, 1818 | The survey of the western and northern borders ofOhio, performed by the state of Ohio and thus conforming to their definition of their border, was adopted by Ohio on this date.[61][146] | |
| February 6, 1818 | Alabama Territory createdTuskaloosa County with a description that inadvertently overlapped withMississippi. It described the border of the county as running "a due west course to, the Tombeckbe river; thence up the same to the Cotton Gin Port".[147] Unknown at the time, the origin of theTombigbee River andCotton Gin Port were in Mississippi. | Change on paper only: |
| June 30, 1818 | Per the terms of theTreaty of Ghent ending theWar of 1812, theUnited Kingdom returnedMoose Island toMassachusetts, and the United States returnedCampobello Island,Deer Island, andGrand Manan Island to the United Kingdom, all of which were captured from the other side during the war.[148] | no change to map |
| July 13, 1818 | The survey of theGeorgia–Tennessee border was completed.[12][149] | |
| December 3, 1818 | The half ofIllinois Territory south of 42°30′ north was admitted as the twenty-first state,Illinois. The remainder of the territory, along with the unorganized territory that was recently part ofIndiana Territory, was assigned toMichigan Territory.[126][150] |  |
| Date | Event | Change map |
|---|---|---|
| January 30, 1819 | TheTreaty of 1818 went into effect, setting49° north as the border with theUnited Kingdom west of theLake of the Woods, and also establishing theOregon Country as a shared region with the United Kingdom.[2][151][152] Oregon Country had no defined northern limit, but it can be assumed that it did not encroach much uponRussian-held lands; this map uses the later-established line at54°40′ north for simplicity. |  Northwestern North America:  |
| July 4, 1819 | Arkansaw Territory was organized from the southern slice ofMissouri Territory.[v][153][154] |  |
| December 14, 1819 | Alabama Territory was admitted as the twenty-second state,Alabama.[143][155] The statehood act provided for a survey of the southern part of the border withMississippi, which was intended to be north–south, for adjustment if it was discovered to encroach upon Mississippi's established counties; it was later discovered to do so. |  |
| March 15, 1820 | As part of theMissouri Compromise, theDistrict of Maine, the northern and separate part ofMassachusetts, was admitted as the twenty-third state,Maine.[156][157] |  |
| April 21, 1820 | This is the earliest known date of the name "Arkansas Territory" being officially used instead of "Arkansaw Territory".[158] |  |
| May 12, 1820 | The border betweenKentucky andTennessee was established. To make up for the fact that the border between theCumberland Gap and theTennessee River veered north as much as almost 10 miles from36°30′ north, a new survey was conducted starting at that latitude on theMississippi River and moving east to the Tennessee River, hence guaranteeing this last bit of border would fit the original ideal.[41] |  |
| July 19, 1820 | The overlap of the longitudinal southern border betweenAlabama andMississippi was resolved, as per the act admitting Alabama as a state, because the provisional border encroached on Mississippi.[143][159] As the result of a survey, the southern border terminus was moved about 3.8 miles to the east, which changed the border up to the then-northwest corner of Alabama's Washington County. The date when this happened is unclear; the sources available give either an unpublished report dated May 29, 1820, or the completion of the demarcation of the new line on July 19, 1820. | Change on paper only: |
| December 19, 1820 | Alabama redefined some county borders, ending its erroneous overlap ofMississippi created on February 6, 1818.[160] | Change on paper only: |
| 1821 | The surveys ofNorth Carolina's borders withGeorgia[161] andTennessee[19] were completed by 1821. | |
| February 22, 1821 | TheAdams–Onís Treaty withSpain took effect.[118] The many changes included:
|  Disputes:  |
| July 10, 1821 | East Florida was formally transferred to the United States bySpain.[162] |  |
| July 17, 1821 | West Florida was formally transferred to the United States bySpain.[162] |  |
| July 25, 1821 | The survey of theIllinois–Indiana border was completed.[163] | |
| August 10, 1821 | The southeastern corner ofMissouri Territory was admitted as the twenty-fourth state,Missouri, the rest becoming unorganized territory.[x][134][164] |  |
| March 30, 1822 | The formerEast Florida andWest Florida were organized asFlorida Territory.[165][166] |  |
| 1823 | The survey of the westernMissouri border south of theKansas River was completed by 1823.[134] | |
| May 26, 1824 | The half ofArkansas Territory west of a line south from a point 40 miles west ofMissouri's western border was returned to unorganized territory.[154][167] |  |
| January 12, 1825 | Atreaty with theRussian Empire established54°40′ north as the northern border ofOregon Country for American purposes; a separate treaty created the same border between Russia and theUnited Kingdom.[168] As this was likely thede facto border anyway, the region is already mapped with this line. | no change to map |
| 1826 | The survey of theGeorgia–Alabama border was run and accepted by Georgia in 1826; it would not be accepted by Alabama until January 24, 1840.[12] | |
| 1827 | The survey of theIndiana–Michigan Territory border was completed by 1827.[109] | |
| May 6, 1828 | A treaty with theCherokee moved the western border ofArkansas Territory, returning part of it to unorganized territory.[y][154][170] |  |
| February 28, 1829 | The survey of theMaine–New Hampshire border was approved by Maine on this date; it would be approved by New Hampshire on July 1, 1829. | |
| 1831 | The survey of the westernArkansas Territory border north ofFort Smith was completed by 1831.[154] | |
| January 20, 1831 | KingWilliam I of the Netherlands, having been asked per theTreaty of Ghent to arbitrate the disputed border betweenMaine and theUnited Kingdom, rendered his decision: since reconciling the treaty with the maps given was too difficult, he drew a compromise line. The British government accepted it, but Maine protested, and on January 19, 1832, the American government rejected it.[171] | no change to map |
| 1832 | The survey of theIllinois–Michigan Territory border was completed by 1832.[126] | |
| July 9, 1832 | The region ofNew Hampshire north of theConnecticut Lakes, which was disputed with theUnited Kingdom, declared independence as theRepublic of Indian Stream.[172] While tiny, it does appear to have maintained some control over its territory. | Disputes: |
| June 28, 1834 | Michigan Territory gained a large parcel of land from unorganized territory, extending west to theMissouri River andWhite Earth River.[123][173] |  |
| August 5, 1835 | TheRepublic of Indian Stream recognized the jurisdiction ofNew Hampshire, thus ending its claimed independence. The date given is of a communication sent toBritish authorities;[174] other sources note a resolution passed by the citizens of Indian Stream on April 2, 1836.[172] | Disputes: |
| June 15, 1836 | Arkansas Territory was admitted as the twenty-fifth state,Arkansas.[154][175] |  |
| July 3, 1836 | Wisconsin Territory was organized from the western bulk ofMichigan Territory.[z][176][177] The two large peninsulas between theGreat Lakes remained in Michigan Territory; theupper peninsula was included in exchange for the territory abandoning its claim to theToledo Strip. The territory initially rejected this plan, but would accept it on December 14. |  |
| December 14, 1836 | Michigan Territory agreed to abandon its claim to theToledo Strip, ending its dispute withOhio.[178] |  |
| January 26, 1837 | Michigan Territory was admitted as the twenty-sixth state,Michigan.[123][179] |  |
| March 28, 1837 | ThePlatte Purchase, obtained from several nations including thePotawatomi,Iowa,Missouria,Otoe,Sauk andMeskwaki,[180] transferred some land from unorganized territory to northwestMissouri, extending its northern border west to theMissouri River.[134][181] |  |
| November 9, 1837 | The survey of theTennessee–Mississippi border was approved byTennessee.[182] | |
| July 3, 1838 | Iowa Territory was organized fromWisconsin Territory west of theMississippi River.[183][184] |  |
| 1839 | The surveys of theAlabama–Tennessee border,[143] and the remaining few miles of theArkansas–Louisiana border,[121] were completed by 1839. | |
| February 11, 1839 | Missouri claimed an area north of itsborder withIowa Territory, initiating the long dispute known as theHoney War.[185] |  |
| May 21, 1840 | Surveying conducted along the border withTexas concluded that the area claimed byArkansas forMiller County belonged to Texas.[186] |  |
| November 10, 1842 | TheWebster–Ashburton Treaty defined the border with theUnited Kingdom east of theRocky Mountains.[187][188] One source also mentions it very slightly altering the maritime border between Michigan and Wisconsin Territory.[189] The treaty resolved the disputes over the northern borders ofMaine andNew Hampshire,[aa] the northeastern border ofWisconsin Territory,[ab] andSugar Island withMichigan. The border betweenNew York andVermont on the one side, and the United Kingdom on the other, was clarified by the treaty. In 1816, construction began on an unnamed fort nicknamed "Fort Blunder" on a peninsula inLake Champlain that, while south of the surveyed border, was discovered to be north of45° north, which was the border set by theTreaty of Paris and thus in British territory. Consequently, construction on the fort was abandoned. The Webster–Ashburton Treaty specified that section of the border was to follow the surveyed line, rather than the exact parallel, thus moving the fort's area into the United States, and a new fort,Fort Montgomery, would be built on the spot in 1844.[191] As the earlier line was surveyed, even though it did not match the definition, it was deemed to be the legitimate border. |  Disputes:  |
| July 5, 1843 | Local settlers created aprovisional government forOregon Country. While not official, it did maintain some jurisdiction over the area.[192] | Unofficial change: |
| March 3, 1845 | Florida Territory was admitted as the twenty-seventh state,Florida.[166][193] |  |
| Date | Event | Change map |
|---|---|---|
| December 29, 1845 | TheRepublic of Texas wasannexed and admitted as the twenty-eighth state,Texas, extending the United States southwest to theRio Grande.[194][195] All of Texas was claimed byMexico. While many sources state that Mexico recognized the independence of the eastern portion of Texas,the treaties were rejected by the Mexican government. Texas formally handed over sovereignty to the United States in a ceremony on February 19, 1846.[196] The annexation led to the beginning of theMexican–American War a few months later.[196] |  Disputes:  |
| June 15, 1846 | TheOregon Treaty established49° north west of theLake of the Woods as the continental border (so it did not includeVancouver Island) with land held by theUnited Kingdom. The sharing ofOregon Country ended, and the United States portion became unorganized territory.[197] The treaty was vague on which strait should be the border between Vancouver Island and the continent, thus causing a dispute over ownership of theSan Juan Islands.[198] It specified "through the middle of the said channel and of Fuca Straits, to the Pacific Ocean". |  Northwestern North America:  Disputes:  |
| September 22, 1846 | Following thecapture on August 18, 1846, ofSanta Fe, the capital of theMexican territory ofSanta Fe de Nuevo México,[199] a code of laws known as theKearny Code was created for the area.[200][201] The region overlapped withTexas' claim, though Texas had little to no control over the area outside of its eastern quarter. | Unofficial change: |
| December 28, 1846 | The portion ofIowa Territory south of 43°30′ north and east of theBig Sioux River was admitted as the twenty-ninth state,Iowa. The remainder became unorganized territory.[184][202] |  |
| March 13, 1847 | TheDistrict of ColumbiaretrocededAlexandria County back toVirginia.[89] Congress passed the act on July 9, 1846,[203] residents of Alexandria County were proclaimed by the president to have agreed to it on September 7, 1846,[204] and Virginia took possession of the land on this date.[205] |  |
| June 28, 1847 | The survey of the border with theUnited Kingdom east of theGreat Lakes came into force.[206] | |
| November 5, 1847 | The survey of theMichigan–Wisconsin Territory border was submitted to the United States Congress on this date.[177][207] | |
| February 15, 1848 | The survey of theArkansas–Missouri border was ratified by the United States Congress.[154][208] | |
| May 29, 1848 | The southern bulk ofWisconsin Territory was admitted as the thirtieth state,Wisconsin.[ac] The remainder became unorganized territory.[177][209] However, the citizens of the remainder decided to continue on with a civil government, and even elected a delegate to the United States House of Representatives who would be seated on January 15, 1849, essentially making this region ade facto continuation of Wisconsin Territory.[210] |  |
| July 4, 1848 | TheTreaty of Guadalupe Hidalgo ended theMexican–American War and ceded alarge parcel of land fromMexico, consisting of its territories ofAlta California andSanta Fe de Nuevo México, and its claim toTexas.[ad][212] A border dispute began over a disagreement about the southern border of Santa Fe de Nuevo México.[211] |  Disputes:  |
| August 14, 1848 | Oregon Territory was organized from the unorganized territory that was formerly part ofOregon Country.[213][214] |  |
| February 13, 1849 | The border dispute betweenIowa andMissouri known as theHoney War wasresolved by theSupreme Court of the United States. The resulting border was theSullivan Line that existed before the dispute, roughly splitting the two claims.[215] |  |
| March 3, 1849 | Minnesota Territory was organized from the region that had been operating asde factoWisconsin Territory, and unorganized territory east of theMissouri andWhite Earth Rivers.[216][190] |  |
| March 12, 1849 | A local government formed theState of Deseret and claimed a vast portion of the southwest, including most of theMexican Cession. Though it petitioned to be admitted to the United States, the proposal was rejected and, in 1850,Utah Territory was formed instead.[217] The claimed area overlapped slightly withTexas' claimed area, as well as part ofOregon Territory. | Unofficial change: |
| November 24, 1849 | Texas, with consent from the United States Congress,[218] extended its border withLouisiana from the west bank of theSabine River to the middle of the river.[218][219][220][221] | no change to map |
| September 9, 1850 | The western portion of theMexican Cession was admitted as the thirty-first state,California.[ae][3][222] The portion of the remainder north of37° north and west of the summit of theRocky Mountains was organized asUtah Territory.[223][224] Part of Utah Territory overlapped with the portion ofTexas that would be purchased on December 13, 1850, but the law authorizing the purchase was passed on this day, so the borders of Utah Territory assumed that the purchase would go through. |  |
| December 9, 1850 | TheUnited Kingdom ceded less than one acre of underwater rock for alighthouse inLake Erie nearBuffalo, New York; although it was on Middle Reef, it was named for the nearby Horseshoe Reef.[225] It was surrounded by British waters, thus creating a form of enclave.[206] |  |
| December 13, 1850 | The federal government purchased the western claims ofTexas.[af][195]New Mexico Territory was organized from the part of this land east of theRio Grande, along with the remaining unorganized territory from theMexican Cession.[ag][227][226] New Mexico Territory included all of the area that had been governed under theKearny Code. |  |
| January 3, 1851 | The survey of theIowa–Missouri border was approved by theSupreme Court of the United States.[134][228] | |
| April 5, 1851 | TheState of Deseret dissolved itself, its functions and territory largely having been superseded byUtah Territory.[229] | Unofficial change: |
| 1852 | The survey of the northernIowa border was completed by 1852.[184] | |
| March 2, 1853 | Washington Territory was organized from the half ofOregon Territory north of46° north and theColumbia River.[230][231] |  |
| May 30, 1854 | Kansas Territory was organized from unorganized territory north of37° north, andNebraska Territory was organized north of40° north.[232][233][234] Much of the remaining unorganized territory, east of100° west, became known asIndian Territory, designated as a place to resettle Indian tribes. A small strip between theTexas Panhandle and Kansas Territory was unclaimed because it fell south of Kansas Territory's border but north of36°30′ north, which had been established in theMissouri Compromise as the northern limit of slavery, and thus Texas could not have it. This became known as thePublic Land Strip, or sometimes "No Man's Land".[235] |  |
| June 30, 1854 | The United States purchased a large parcel fromMexico known as theGadsden Purchase, as it offered a much better route for a southern transcontinental railroad.[ah][236][237] This resolved the border dispute, since the disputed land was included in the purchase.[211] |  Disputes:  |
| August 4, 1854 | The recently obtainedGadsden Purchase was assigned toNew Mexico Territory.[226][238] |  |
| 1855 | The survey of part of theKansas Territory–Nebraska Territory border was completed by 1855.[233] | |
| January 11, 1855 | Boston Corner was transferred fromMassachusetts toNew York because it was inaccessible from the rest of the Massachusetts.[239][240][241] |  |
| March 6, 1855 | The Supreme Court ruled in favor ofFlorida in a border dispute withGeorgia, setting the state line alongMcNeil's line.[242] | no change to map |
| August 1, 1856 | The survey of the border withMexico was submitted to theUnited States Congress.[236][243] | |
| October 28, 1856 | Baker Island andJarvis Island were claimed under theGuano Islands Act.[4] | Pacific Ocean: |
| 1857 | The survey of the southernKansas Territory border was completed by 1857.[233] | |
| May 11, 1858 | The eastern half ofMinnesota Territory was admitted as the thirty-second state,Minnesota.[ai] The remainder became unorganized territory.[190][244] |  |
| August 31, 1858 | Navassa Island was claimed under theGuano Islands Act.[4] | Caribbean Sea: |
| December 3, 1858 | Howland Island was claimed under theGuano Islands Act.[4] | Pacific Ocean: |
| 1859 | The survey of more of theKansas Territory–Nebraska Territory border was completed by 1859.[233] | |
| February 14, 1859 | The western half ofOregon Territory was admitted as the thirty-third state,Oregon.[aj] The remainder was transferred toWashington Territory.[214][245] |  |
| July 6, 1859 | A team of surveyors created the "Middleton Offset", a small notch in the border betweenKentucky andTennessee. It is unknown exactly why this was done, though one theory is a local landowner wanted his property in Tennessee.[246][247] |  |
| September 6, 1859 | Johnston Atoll was claimed under theGuano Islands Act,[4] though it had been claimed byHawaii in 1858.[248] | Pacific Ocean: |
| November 7, 1859 | A local government was set up encompassing parts of the territories ofKansas,Nebraska,New Mexico,Utah, andWashington, with the name ofJefferson Territory.[ak] While never recognized by the federal government, it generally held control over the area untilColorado Territory was established, which adopted most of its laws.[249] | Unofficial change: |
| December 27, 1859 | Enderbury Island,McKean Island,Phoenix Island, andStarbuck Island were claimed under theGuano Islands Act.[4] | Pacific Ocean: |
| December 29, 1859 | Christmas Island andMalden Island were claimed under theGuano Islands Act.[4] | Pacific Ocean: |
| 1860 | The survey of the northern border ofTexas was completed by 1860.[195] | |
| February 8, 1860 | Texas createdGreer County, claiming part ofIndian Territory based on a different understanding from the federal government of which fork of theRed River was the border between the two.[250] Atafu,Birnie Island,Butaritari,Caroline Island,Fanning Island,Flint Island,Gardner Island,Canton Island,Kingman Reef,Manihiki,Marakei,Nukunono,Palmyra Atoll,Penrhyn,Pukapuka,Rakahanga,Swains Island,Sydney Island,Vostok Island, andWashington Island were all claimed under theGuano Islands Act.[4] Many additional islands were listed as bonded on this date, but based on the coordinates they were either phantoms or duplicates. In addition,Sarah Ann Island was claimed, which may have existed and would be sighted as late as 1917, but has since disappeared.[251] |  Pacific Ocean:  |
| Date | Event | Change map |
|---|---|---|
| December 20, 1860 | In response to theelection ofAbraham Lincoln,South Carolina proclaimed its secession from the Union, withdrawing from the Congress of the United States.[252] |  Disputes:  |
| January 9, 1861 | Mississippi proclaimed its secession from the Union, withdrawing from Congress.[252] |  Disputes:  |
| January 10, 1861 | Florida proclaimed its secession from the Union, withdrawing from Congress.[252] |  Disputes:  |
| January 11, 1861 | Alabama proclaimed its secession from the Union, withdrawing from Congress.[252] |  Disputes:  |
| January 19, 1861 | Georgia proclaimed its secession from the Union, withdrawing from Congress.[252] |  Disputes:  |
| January 26, 1861 | Louisiana proclaimed its secession from the Union, withdrawing from Congress.[252] However, the1st and2nd congressional districts, aroundNew Orleans, maintained representation in Congress. |  Disputes:  |
| January 29, 1861 | The bulk ofKansas Territory east of25° west from Washington was admitted as the thirty-fourth state,Kansas. The remainder became unorganized territory.[233][253] |  |
| February 8, 1861 | TheConfederate States of America was formed by representatives of the seceded states ofAlabama,Georgia,Florida,Louisiana,Mississippi, andSouth Carolina.[254] The survey of theFlorida–Georgia border was approved by Florida on this date;[255] it would be approved by Georgia on December 13, 1866,[256] and would be acknowledged by the federal government on April 9, 1872.[257] | Disputes: |
| February 28, 1861 | Colorado Territory was organized from portions ofNebraska Territory,New Mexico Territory, andUtah Territory, along with unorganized territory.[al][259][258] |  |
| March 2, 1861 | Texas proclaimed its secession from the Union and was admitted to theConfederate States,[252][260] withdrawing from Congress. Dakota Territory was organized fromNebraska Territory and the unorganized territory north of it.[am] Nebraska Territory's western border was moved to 33° west from Washington, gaining small portions ofUtah Territory andWashington Territory.[234][261][262]Nevada Territory was organized fromUtah Territory west of39° west from Washington.[263][264] |  Disputes:  |
| March 28, 1861 | Representatives in the southern half ofNew Mexico Territory proclaimed an independentArizona Territory south of34° north.[265] | Disputes: |
| April 17, 1861 | Following theBattle of Fort Sumter and PresidentAbraham Lincoln'scall for troops to respond,Virginia proclaimed its secession from the Union, withdrawing from Congress.[252] However, the1st (along theEastern Shore),7th (nearWashington, D.C.), and10th,11th, and12th (in the northwest of the state) congressional districts maintained representation in Congress. |  Disputes:  |
| May 6, 1861 | Arkansas proclaimed its secession from the Union, withdrawing from Congress.[252] |  Disputes:  |
| May 7, 1861 | Virginia was admitted to theConfederate States.[266] | Disputes: |
| May 16, 1861 | Kentuckydeclared itself neutral in theAmerican Civil War. | no change to map |
| May 20, 1861 | Arkansas was admitted to theConfederate States.[267] North Carolina proclaimed its secession from the Union, withdrawing from Congress.[252] |  Disputes:  |
| May 21, 1861 | North Carolina was admitted to theConfederate States. The law admitting the state required a presidential proclamation before it was to take effect,[268] which sources say took place on this date;[269] the only primary source found so far is a statement fromJefferson Davis on July 20 stating that the proclamation had been made.[270] | Disputes: |
| June 6, 1861 | Robert Williamson Steele, governor ofJefferson Territory, declared the territory disbanded and handed over the government to the first governor ofColorado Territory.[249] | Unofficial change: |
| June 8, 1861 | Tennessee proclaimed its secession from the Union, withdrawing from Congress.[252] However, the2nd,3rd, and4th congressional districts in the central part of the state maintained representation in Congress. |  Disputes:  |
| June 25, 1861 | The federal government recognized theRestored Government of Virginia inWheeling as the legitimate government ofVirginia.[271] |  |
| July 2, 1861 | Tennessee was admitted to theConfederate States.[272] | Disputes: |
| August 1, 1861 | Following Confederate victory in theFirst Battle of Mesilla,Arizona Territory was proclaimed as part of theConfederate States.[273] | Disputes: |
| September 13, 1861 | Following the Confederate occupation ofColumbus,Kentucky, on September 3, 1861, the state abandoned neutrality and aligned with the Union government.[274] | no change to map |
| October 31, 1861 | A splinter government inNeosho, Missouri, proclaimed the secession of the state from the United States.[252] | Disputes: |
| November 20, 1861 | A convention inRussellville, Kentucky, proclaimed the formation of a splinter government inBowling Green and the secession ofKentucky from the United States.[252] | Disputes: |
| November 28, 1861 | The splinterNeosho government ofMissouri was admitted to theConfederate States. The Confederate States never held much power over the state, but it was given full representation in the legislature.[275] | Disputes: |
| December 10, 1861 | The splinterBowling Green government ofKentucky was admitted to theConfederate States. The Confederate States never held much power over the state, but it was given full representation in the legislature.[276] | Disputes: |
| December 21, 1861 | TheConfederate States ratified treaties with theOsage, and theSeneca andShawnee.[277][278] | Disputes: |
| December 23, 1861 | TheConfederate States ratified treaties with theCherokee, granting them a delegate to theCongress of the Confederate States, and with theSeminole, granting them a delegate to be shared with theCreek.[277][278] | Disputes: |
| December 31, 1861 | TheConfederate States ratified treaties with theChoctaw andChickasaw, granting them a delegate in theCongress of the Confederate States; with theComanche; with theCreek, granting them a delegate to be shared with theSeminole; and theQuapaw.[277][278] | Disputes: |
| March 1, 1862 | A decree by theSupreme Court of the United States took effect, modifying the border betweenMassachusetts andRhode Island.[an][14][279] |  |
| April 15, 1862 | Palmyra Atoll was annexed byHawaii, and the American claim falls dormant.[280] | Pacific Ocean: |
| July 14, 1862 | The slice ofUtah Territory west of 38°west from Washington was transferred toNevada Territory.[264][281] |  |
| December 30, 1862 | TheSwan Islands were claimed under theGuano Islands Act.[4] | Caribbean Sea: |
| February 24, 1863 | Arizona Territory was organized from the half ofNew Mexico Territory west of32° west from Washington.[282][283] |  |
| March 3, 1863 | Idaho Territory was organized from the parts ofDakota Territory andNebraska Territory west of27° west from Washington, and the half ofWashington Territory east of theSnake River and a line north from the mouth of theClearwater River.[284][285] |  |
| March 4, 1863 | Because of a disruption in voting and low turnout, no one was allowed to take the seats in theUnited States House of Representatives held by the Unionist areas ofLouisiana,Tennessee, andVirginia, effectively expelling the states.[286] |  |
| June 20, 1863 | The northwestern counties ofVirginia, represented by theRestored Government of Virginia inWheeling, were split from the rest of Virginia and admitted to the Union as the thirty-fifth state,West Virginia.[ao][288][287] The Restored Government of Virginia was relocated toAlexandria. |  |
| August 5, 1863 | Berkeley County was transferred by the federal government fromVirginia toWest Virginia.[289] |  |
| November 2, 1863 | Jefferson County was transferred fromVirginia toWest Virginia.[290] |  |
| 1864 | The survey of theOregon–Washington Territory border was completed by 1864.[214] | |
| May 26, 1864 | Montana Territory was organized from the northeast third ofIdaho Territory,[ap] and the southeast third of Idaho Territory was transferred toDakota Territory.[aq][291][262][292] |  |
| October 15, 1864 | Malden Island was claimed by theUnited Kingdom.[293] | Pacific Ocean: |
| October 31, 1864 | Nevada Territory was admitted as the thirty-sixth state,Nevada.[264][294] |  |
| May 5, 1865 | TheConfederate States cabinet met inWashington, Georgia, and dissolved.[295] Military surrenders were scattered throughout 1865, but the most important is regarded as that of theArmy of Northern Virginia following theBattle of Appomattox Court House on April 9. | Disputes: |
| Date | Event | Change map |
|---|---|---|
| May 5, 1866 | The slice ofUtah Territory west of37° west from Washington was transferred toNevada.[296] |  |
| July 24, 1866 | Tennessee was readmitted to Congress.[297] |  |
| December 26, 1866 | Starbuck Island was claimed by theUnited Kingdom.[293] | Pacific Ocean: |
| January 18, 1867 | The northwestern corner ofArizona Territory, west of theColorado River and37° west from Washington, was transferred toNevada. The law transferring the land was approved May 5, 1866, but unlike theUtah Territory transfer of that day, this transfer was contingent on the state accepting it.[296][298] |  |
| March 1, 1867 | Nebraska Territory was admitted as the thirty-seventh state,Nebraska.[234][299] |  |
| July 1, 1867 | Canada was formed from several British colonies, includingNew Brunswick, thus inheriting the dispute overMachias Seal Island andNorth Rock. | Disputes: |
| August 28, 1867 | Midway Atoll was claimed.[300] An attempt had been made at the time of its discovery in 1859 to claim it under theGuano Islands Act.[301] | Pacific Ocean: |
| October 18, 1867 | Alaska waspurchased from theRussian Empire and designated theDepartment of Alaska.[ar][303][304] A vague description and lack of quality surveying made the southeastern border withBritish holdingsunclear and disputed.[302] | Northwestern North America: |
| 1868 | The surveys of theColorado Territory–New Mexico Territory[258] and theOregon–Idaho Territory[214] borders were completed by 1868. | |
| June 22, 1868 | Arkansas was readmitted to Congress.[305] |  |
| June 25, 1868 | Florida was readmitted to Congress.[306] |  |
| July 4, 1868 | North Carolina was readmitted to Congress.[307] |  |
| July 9, 1868 | Louisiana andSouth Carolina were readmitted to Congress.[308] Caroline Island was claimed by theUnited Kingdom.[293] |  Pacific Ocean:  |
| July 13, 1868 | Alabama was readmitted to Congress.[309] |  |
| July 25, 1868 | Georgia was readmitted to Congress.[310] Wyoming Territory was organized from portions ofDakota,Idaho, andUtah Territories.[as][312][311] The territory would remain under the jurisdiction of the Dakota Territory until its own government was organized on May 19, 1869.[313] The act organizing Wyoming Territory became law on this date, but it is unclear if the territory could be considered "organized" until May 19, 1869, as the act specifies it was not to take effect until a government is organized; however, all sources use this date as the creation, and most use it for the organization, of the territory. Atiny portion of the Dakota Territory was erroneously left behind on the western side of Wyoming Territory.[314] |  |
| August 12, 1868 | The list of bondedguano island claims mentions "Islands in Caribbean Sea not named" bonded on this date, but it is unknown to what this is referring.[4] | no change to map |
| December 11, 1868 | Serrana Bank was claimed under theGuano Islands Act.[4]Colombia has claimed it throughout its history. | Caribbean Sea: |
| 1869 | The surveys of theCalifornia–Oregon border[3] and the western and southwestern borders ofNebraska[234] were completed by 1869. | |
| March 3, 1869 | Georgia was again expelled from Congress following failures ofReconstruction in the state.[315] |  |
| November 22, 1869 | Bajo Nuevo Bank,Pedro Cays,Quita Sueño Bank, andRoncador Bank were claimed under theGuano Islands Act.[4][300] Except for Pedro Cays,Colombia has claimed them throughout its history. | Caribbean Sea: |
| 1870 | The surveys of the border with theUnited Kingdom west of theRocky Mountains, and the eastern border ofNevada,[264] were completed by 1870. | |
| January 26, 1870 | Virginia was readmitted to Congress.[316] |  |
| February 23, 1870 | Mississippi was readmitted to Congress.[317] |  |
| March 30, 1870 | Texas was readmitted to Congress.[318] |  |
| July 15, 1870 | Georgia was again readmitted to Congress.[319] TheNorth-Western Territory was transferred by theUnited Kingdom toCanada, thus transferring its portion of theAlaska boundary dispute.[320] |  Northwestern North America:  |
| 1871 | The survey of theIdaho Territory–Utah Territory border was completed by 1871.[224] | |
| February 9, 1871 | A small parcel was transferred fromDakota Territory toNebraska following a sudden change in course of theMissouri River.[234][321] |  |
| July 20, 1871 | British Columbia joinedCanada, transferring thedispute over theSan Juan Islands as well as its portion of theAlaska boundary dispute.[322] | Disputes: Northwestern North America:  |
| 1872 | The survey of theKansas–Colorado Territory border was completed by 1872.[233] | |
| October 21, 1872 | Thedispute withCanada over theSan Juan Islands was resolved in the favor of the United States claim.[198] | Disputes: |
| 1873 | Vostok Island was claimed by theUnited Kingdom.[323] The surveys of the remaining borders ofNevada,[264][3] theIdaho Territory–Washington Territory border,[285] and the southern border ofWyoming Territory,[311] were completed by 1873. | Pacific Ocean: |
| February 17, 1873 | Thesmall portion ofDakota Territory that was left behind whenWyoming Territory was created was transferred toMontana Territory.[314][324] |  |
| 1874 | The surveys of theNebraska–Dakota Territory border,[234] theNew Mexico Territory–Public Land Strip border,[226] and the westernWyoming Territory border[285] were completed by 1874. | |
| 1875 | The survey of theArizona Territory–New Mexico Territory border was completed by 1875.[226] | |
| August 1, 1876 | Colorado Territory was admitted as the thirty-eighth state,Colorado.[258][325] |  |
| 1877 | The surveys of the border withCanada between theRocky Mountains andGreat Lakes, and theDakota Territory–Wyoming Territory border[262] were completed by 1877. | |
| August 13, 1877 | TheUnited Kingdom created theBritish Western Pacific Territories, includingAtafu andNukunono.[326] | Pacific Ocean: |
| 1879 | The survey of theColorado–Utah Territory border was completed by 1879.[258] | |
| March 3, 1879 | The border across theChesapeake Bay betweenMaryland andVirginia was decided via arbitration. It is unknown if any land actually changed hands.[13] | too vague to map |
| September 8, 1879 | Arenas Key, claimed byMexico; theMorant Cays; andSerranilla Bank, claimed byColombia, were claimed by the United States under theGuano Islands Act;[4] according to the Office of Insular Affairs, Serranilla Bank was claimed again on September 13, 1880.[300] | Caribbean Sea: |
| 1880 | The survey of the northern border ofWyoming Territory was completed by 1880.[311] | |
| April 7, 1880 | A very small area ofFair Haven, Vermont, was transferred toNew York in response to a change in the course in thePoultney River.[32][327] | too small to map |
| September 13, 1880 | Western Triangle Island, claimed byMexico, was claimed by the United States under theGuano Islands Act.[4] The list of bonded claims also mentions a "De Anes" island claimed on this date, with coordinates matchingIslade Aves; however, the same list points out that the claim to "Aves Island" was found to be invalid. | Caribbean Sea: |
| 1881 | Flint Island was claimed by theUnited Kingdom.[328] | Pacific Ocean: |
| May 23, 1882 | The area between43° north and theKeya Paha andNiobrara Rivers was transferred fromDakota Territory toNebraska.[262][329] The act was passed in Congress on March 28 and accepted by the Nebraska legislature on this date.[330] |  |
| June 1, 1882 | TheMorant Cays andPedro Cays were annexed by theUnited Kingdom toJamaica; it appears they were no longer claimed by the United States after this.[331] | Caribbean Sea: |
| May 17, 1884 | TheDepartment of Alaska was organized into theDistrict of Alaska.[332] | Northwestern North America: |
| June 21, 1884 | TheAlacrans Islands, claimed byMexico, were claimed under theGuano Islands Act.[4] | Caribbean Sea: |
| 1885 | The survey of theDakota Territory–Montana Territory was completed by 1885.[291] | |
| March 15, 1888 | Fanning Island was annexed by theUnited Kingdom; it appears the island was no longer claimed by the United States after this.[333] | Pacific Ocean: |
| March 17, 1888 | Christmas Island was claimed by theUnited Kingdom.[293] | Pacific Ocean: |
| October 26, 1888 | TheCook Islands became a protectorate of theUnited Kingdom, thus initiating a claim on the atolls ofManihiki,Penrhyn,Pukapuka, andRakahanga.[293] | Pacific Ocean: |
| May 29, 1889 | Washington Island was annexed by theUnited Kingdom; it appears the island was no longer claimed by the United States after this.[333] | Pacific Ocean: |
| June 3, 1889 | Jarvis Island was claimed by theUnited Kingdom.[293] | Pacific Ocean: |
| June 26, 1889 | Sydney Island was claimed by theUnited Kingdom.[293] | Pacific Ocean: |
| June 29, 1889 | Phoenix Island was claimed by theUnited Kingdom.[293] | Pacific Ocean: |
| July 10, 1889 | Birnie Island was claimed by theUnited Kingdom.[293] | Pacific Ocean: |
| November 2, 1889 | Dakota Territory was split in half along the "seventh standard parallel north", a few miles south of46° north, and admitted as the thirty-ninth state,North Dakota, and the fortieth state,South Dakota.[262][334] |  |
| November 8, 1889 | Montana Territory was admitted as the forty-first state,Montana.[291][334] |  |
| November 11, 1889 | Washington Territory was admitted as the forty-second state,Washington.[231][334] |  |
| May 2, 1890 | Oklahoma Territory wasorganized from thePublic Land Strip and the western half ofIndian Territory, except for theCherokee Outlet, which would be added later upon cession from theCherokee.[at][336][335] |  |
| July 3, 1890 | Idaho Territory was admitted as the forty-third state,Idaho.[285][337] |  |
| July 10, 1890 | Wyoming Territory was admitted as the forty-fourth state,Wyoming.[311][338] |  |
| 1892 | The survey of theNorth Dakota–South Dakota border was completed by 1892.[262] | |
| March 8, 1892 | TheGilbert Islands became a protectorate of theUnited Kingdom, thus initiating a claim onButaritari andMarakei.[293] No record of a United States claim exists after this point, so it is assumed this is when the claim fell dormant. | Pacific Ocean: |
| May 28, 1892 | Gardner Island was claimed by theUnited Kingdom.[293] | Pacific Ocean: |
| 1893 | The survey of the modifiedNebraska–South Dakota border completed by 1893.[234] | |
| September 16, 1893 | Per a treaty with theCherokee, the federal government purchased theCherokee Outlet in theIndian Territory and opened it to settlement, transferring it toOklahoma Territory as provided in theOklahoma Organic Act.[335][339] |  |
| November 17, 1894 | TheAlacrans Islands,Arenas Key, andWestern Triangle Island were stricken from the list of claimedguano islands.[4] | Caribbean Sea: |
| January 4, 1896 | Utah Territory was admitted as the forty-fifth state,Utah.[224][340] |  |
| March 16, 1896 | The dispute between the federal government, on behalf ofOklahoma Territory, andTexas overGreer County was resolved in favor of the federal claim.[250] |  |
| July 24, 1897 | To account for an earlier shift in the course of theMissouri River, an island was transferred fromNebraska toSouth Dakota.[341] |  |
| Date | Event | Change map |
|---|---|---|
| August 12, 1898 | TheRepublic of Hawaii was annexed.[342] The ceremony to transfer sovereignty occurred on this date; theact was signed on July 7, 1898.[343]Johnston Atoll was not included with Hawaii, nor wasSikaiana Atoll, which had been ceded to Hawaii in 1856 by its residents and approved byKing Kamehameha IV. However, the annexation was based on the islands named in a report of the Hawaiian Commission, which omitted Sikaiana.[344][345] | Pacific Ocean: |
| 1899 | The survey of part of theOklahoma Territory–Indian Territory border was completed by 1899.[335] | |
| January 17, 1899 | Wake Island was claimed.[346] | Pacific Ocean: |
| April 11, 1899 | Guam,Porto Rico, and, on agreed payment of $20 million, thePhilippines wereceded bySpain following theSpanish–American War.[347] The Philippines were claimed by theFirst Philippine Republic. The ceded region for the Philippines included the island ofPalmas, which was administered bythe Netherlands. This overlap would not be noticed until January 21, 1906.[348] While the United States occupiedCuba for a time, it was not ceded nor claimed. | Pacific Ocean: Caribbean Sea:  |
| February 16, 1900 | The United States took ownership of theSamoan Islands east of 171° west, per the terms of theTripartite Convention.[349] | Pacific Ocean: |
| February 19, 1900 | The newly acquired Samoan islands were established asNaval Station, Tutuila. It included all of the islands granted by theTripartite Convention, though formal cession of the islands by local authorities would take place later in 1900 and 1904.[349] | Pacific Ocean: |
| April 12, 1900 | Porto Rico wasorganized into a civil territory.[350] | no change to map |
| April 17, 1900 | The island ofTutuila wasformally ceded to the United States and added toNaval Station, Tutuila.[349][351] As the United States had already claimed the island on February 19, 1900, no change is mapped. Thetreaty would be ratified by Congress on February 20, 1929. | |
| June 14, 1900 | The formerRepublic of Hawaii wasorganized intoHawaii Territory.[352][353] | Pacific Ocean: |
| 1901 | The survey of theUtah–Arizona Territory border was completed by 1901.[224] | |
| March 3, 1901 | The transfer for a thin sliver ofBristol, Tennessee, toBristol, Virginia, was approved by Congress after having been approved by both states.[354][355] The location of the border along Main Street (now State Street) between the two cities was either the northern sidewalk of the street, or down the middle of the street; Tennessee's cession of the northern half of the street laid the issue to rest. | too small to map |
| March 23, 1901 | The president of theFirst Philippine Republic,Emilio Aguinaldo, was captured, and the republic was dissolved. On this same date several islands,Cagayan de Sulu andSibutu among them, werepurchased fromSpain and assigned to the Philippines, which was then being governed as a U.S.insular area. The borders specified in theTreaty of Paris of 1898 had excluded these islands; the new treaty simply ceded "any and all islands belonging to the Philippine Archipelago".[356][357] | Pacific Ocean: |
| July 1, 1902 | ThePhilippines wereorganized.[358] | no change to map |
| October 20, 1903 | TheAlaska boundary dispute withCanada was resolved, generally in favor of the United States claim.[302] | Northwestern North America: |
| December 10, 1903 | Land along southernGuantánamo Bay wasleased in perpetuity fromCuba for use as anaval base;[359] the treaty took effect February 23, 1903, and the formal handover occurred on this date.[360] | no change to map |
| May 4, 1904 | The United States took ownership of thePanama Canal Zone. At this stage, only the most basic borders were defined; it was a zone surrounding the canal on each side for five miles, but excluded the cities ofColón andPanama City, which remained exclaves ofPanama, as well as the water for their harbors.[361] TheHay–Bunau-Varilla Treaty granting it to the United States was ratified on February 26, 1904.[362] A formal border agreement, which also gave the Canal Zone some land and a lighthouse in northwest Colón, would be ratified on June 15, 1904.[363][364] | Caribbean Sea: |
| July 16, 1904 | TheManuʻa islands wereformally ceded to the United States and added toNaval Station, Tutuila.[349] As the United States had already claimed the islands on February 19, 1900, no change is mapped. Thetreaty would be ratified by Congress on February 20, 1929. | no change to map |
| December 12, 1904 | The "Taft Agreement" was made withPanama on December 3, with one of its sections refining the maritime border of the harbor ofPanama City and thePanama Canal Zone.[365][366] It became effective December 12. | |
| February 10, 1905 | The border betweenArkansas andIndian Territory was slightly adjusted nearFort Smith, Arkansas, transferring a small amount of land on the east side of thePoteau River to Arkansas.[367][368] |  |
| 1906 | The survey ofIdaho–Montana border was completed by 1906.[285] | |
| September 26, 1907 | New Zealand became largelyindependent from theUnited Kingdom,[369] inheriting the claim on the atolls ofManihiki,Penrhyn,Pukapuka, andRakahanga. | Pacific Ocean: |
| November 16, 1907 | Indian Territory andOklahoma Territory were combined and admitted as the forty-sixth state,Oklahoma.[335][370] |  |
| April 11, 1908 | A border treaty with theUnited Kingdom on behalf ofCanada redefined the maritime borders between the United States and Canada.[371] Among other changes, this "de-enclaved"Horseshoe Reef Lighthouse inLake Erie by making the water around it contiguous with the water on the American side of the border.[206][372] | no change to map |
| January 1, 1909 | The newConstitution of Michigan included some area ofWisconsin within its definition ofMichigan.[373] |  |
| February 21, 1910 | TheSupreme Courtruled that the border betweenMaryland andWest Virginia follows the Deakins Line, surveyed around 1788 but disputed byVirginia and West Virginia.[13][374][375] | |
| August 20, 1910 | A border treaty with theUnited Kingdom on behalf ofCanada addressed a slight uncertainty in the maritime border inPassamaquoddy Bay betweenMaine and Canada.[376][377][378] The border was adjusted to run east of Pope's Folly Island, which previously lay on the border line, and had been the subject of some debate for many years.[379][380] |  |
| February 16, 1911 | The border betweenNew Mexico Territory andTexas was set by theUnited States Congress to the lines surveyed in 1859, but which had never been formally approved by either legislature.[195][381] | |
| July 17, 1911 | Naval Station, Tutuila, was renamedAmerican Samoa;[382] the stationcontinued to operate separate from territorial governance until 1951. | Pacific Ocean: |
| January 6, 1912 | New Mexico Territory was admitted as the forty-seventh state,New Mexico.[226][383] |  |
| February 14, 1912 | Arizona Territory was admitted as the forty-eighth state,Arizona.[282][384] |  |
| August 24, 1912 | TheDistrict of Alaska was reorganized as theAlaska Territory.[385] | Northwestern North America: |
| January 31, 1913 | New Mexico filed suit in theSupreme Court againstTexas over the "Country Club Dispute", questioning whether the present course or the 1850 course of theRio Grande should be their border.[386] |  |
| August 5, 1914 | TheCorn Islands wereleased fromNicaragua for a period of99 years; however, this was not a full transfer of sovereignty, and the islands were never administered as an insular area.[387] | no change to map |
| May 1, 1915 | The borders of thePanama Canal Zone were explicitly defined. Whereas the original definition was a simple corridor surrounding the canal, this treaty specified the actual border. Among the changes this caused were: a slice of Canal Zone was ceded toPanama soPanama City would be connected with the rest of the country; the middle island of the Las Tres Hermanas Islands, which marked the border of Panama City's harbor, was wholly made part of Canal Zone;Gatun Lake and other surrounding waters were formally annexed to the territory; and an area of northwestColón was ceded to Canal Zone.[388][389][390] | Caribbean Sea: |
| May 27, 1915 | Under Article II of the1903 Treaty, the United States expropriated fromPanama several areas around the mouth of theRio Chagres, annexing them to thePanama Canal Zone.[391] | too small to map |
| December 8, 1915 | The United States expropriated fromPanama a triangle of land, which included the historicFort San Lorenzo, between theRio Chagres,Caribbean Sea and thePanama Canal Zone, to which it was annexed.[391] | |
| January 17, 1916 | Navassa Island was formally claimed for lighthouse purposes.[392] | no change to map |
| August 29, 1916 | ThePhilippines werereorganized to provide more autonomous government.[393] | |
| March 31, 1917 | TheDanish Virgin Islands werepurchased fromDenmark[394] and renamed theUnited States Virgin Islands.[395] | Caribbean Sea: |
| July 12, 1918 | The United States expropriated fromPanama 2.6 hectares (6.4 acres) of land atPunta Paitilla inPanama City and annexed it to thePanama Canal Zone.[396] That area was enlarged to about 50 hectares (120 acres) within several months.[391][397][398][399] | too small to map |
| August 21, 1918 | The United States expropriated fromPanama land between theRio Chagres and the Quebrada Majagual and annexed it to thePanama Canal Zone.[391][399] | Caribbean Sea: |
| September 13, 1918 | Minnesota andWisconsin exchanged islands in theMississippi River: Island Seventytwo was transferred toWinona, Minnesota, and Barron's Island was transferred toLa Crosse, Wisconsin.[177][400] |  |
| September 18, 1919 | The island of Largo Remo was annexed to thePanama Canal Zone under the United States right of expropriation in the 1903 Canal Treaty.[401] | Caribbean Sea: |
| June 16, 1920 | 15 hectares (37 acres) on the island ofTaboga Island were annexed to thePanama Canal Zone.[402] | too small to map |
| June 30, 1921 | The "Wedge" dispute betweenDelaware andPennsylvania was resolved in Delaware's favor. The disputed land had generally been administered by Delaware, even electing a member of the Delaware legislature in the mid-19th century,[403] but federal maps had included the land as part of Pennsylvania at least as late as 1900.[404] The states had agreed on a resolution, and it was affirmed by an act of Congress on this date.[405][406] Some sources, both contemporary and modern, note that, in the original process of resurveying the border in 1892, a very thin, horn-shaped region along the arc was transferred from Delaware to Pennsylvania;[403][406][407] however, no federal maps found reflect this, and it is unclear if this transfer actually occurred. |  |
| May 10, 1922 | Kingman Reef was formally annexed.[408] | no change to map |
| October 8, 1923 | Michigan expanded its claim toWisconsin territory, though Wisconsin never lost control over the area.[373] |  |
| November 15, 1923 | TheSwan Islands were claimed byHonduras.[409] | Caribbean Sea: |
| February 1, 1924 | The future area forMadden Lake was annexed to thePanama Canal Zone under the United States right of expropriation in the 1903 Canal Treaty.[364][410][411] | Caribbean Sea: |
| March 4, 1925 | Swains Island was added toAmerican Samoa.[412] | Pacific Ocean: |
| July 17, 1925 | The border withCanada was adjusted in several places.[413][414] The only change to a land border redefined how the border between theLake of the Woods and theRocky Mountains should be considered; previously, the border followed the curve of the parallel between each border monument, while the treaty changed this to straight lines between each monument. Through this, the United States netted a gain of between 30 and 35 acres of land. Because of the extremely small shift, the lack of specific documentation of where the changes occurred, and the lack of any human impact, this change is not mapped. There was also a change to the border in theLake of the Woods; a surveying anomaly caused the previous border to intersect itself several times in the lake, creating enclaves of United States water surrounded by Canadian water. The treaty changed the border to use the southernmost intersection as thenorthwestern point of the Lake of the Woods. Finally, the maritime border in theBay of Fundy was adjusted, netting Canada roughly 9 acres of water. | too small to map |
| March 1, 1926 | TheSupreme Court of the United States resolved the conflict betweenMichigan andWisconsin in the favor ofWisconsin.[373] |  |
| July 29, 1926 | Johnston Atoll was established as a federal bird refuge and placed under the jurisdiction of theUnited States Department of Agriculture.[415] The atoll had originally been claimed by both the United States and Hawaii in 1858, but little activity apart from guano mining had taken place, and it had been largely abandoned for decades.[248] | no change to map |
| November 22, 1926 | TheSupreme Court of the United States defined the border betweenMichigan andWisconsin, transferring all islands south of the Quinnesec Falls on theMenominee River to Wisconsin, and all islands north of the falls toMichigan; it is unknown specifically which islands were transferred in this fashion. However, an error in the border description introduced a small overlap between the two states over several islands inLake Michigan north of theDoor Peninsula.[416] |  |
| July 18, 1927 | The United States expropriated fromPanama another 33 hectares (82 acres) of land on the islands ofTaboga and Taboguilla and annexed them to thePanama Canal Zone.[391] | too small to map |
| October 26, 1927 | Twobancos along theColorado River were ceded fromMexico toArizona.[417][418] | |
| December 5, 1927 | The "Country Club Dispute" betweenNew Mexico andTexas was resolved in Texas's favor.[419] |  |
| April 4, 1928 | TheIsland of Palmas Case was decided in the favor ofthe Netherlands, cedingPalmas to theDutch East Indies.[348] | Pacific Ocean: |
| September 24, 1928 | The United States expropriated fromPanama 3 hectares (7.4 acres) of land atEl Cerro de Doscientos Pies ("200-Foot Hill") near Las Minas Bay and annexed it to thePanama Canal Zone.[391][399] | too small to map |
| March 17, 1930 | The survey of theOklahoma–Texas border was approved by theSupreme Court of the United States.[195] | |
| July 22, 1930 | The United States expropriated fromPanama 25 hectares (62 acres) onJicarita Island and 60 hectares (148 acres) at Punta Morro de Puercos and annexed them to thePanama Canal Zone.[391] | |
| April 15, 1931 | The United States expropriated fromPanama additional areas around the soon-to-be-builtMadden Dam and annexed them to thePanama Canal Zone.[391][399] | Caribbean Sea: |
| May 3, 1932 | The United States adjusted the border atPunta Paitilla in the Canal Zone, returning a small amount of land toPanama. This was the site for a planned new American embassy, which had to be built on foreign soil.[420] | too small to map |
| May 17, 1932 | Porto Rico was renamedPuerto Rico.[421] | Caribbean Sea: |
| December 13, 1932 | The Mangsee Islands and seven of theTurtle Islands wereceded by theUnited Kingdom fromNorth Borneo to thePhilippines. The islands were supposed to be included in the 1900 transfer of islands fromSpain to the United States. Per the terms of the treaty, the United Kingdom continued to administer the islands until requested, and, after the Philippines' independence, the Philippine government made such a request and took control.[422] | Pacific Ocean: |
| May 29, 1933 | TheSupreme Court of the United Statesruled that the border betweenNew Hampshire andVermont was the low water mark of the west bank of theConnecticut River; Vermont had sought to have the border placed in the middle of the river.[46] | no change to map |
| November 13, 1933 | A treaty created theRio Grande Rectification Project, which, from 1935 to 1938, straightened and stabilized the path of theRio Grande through theEl Paso–Juárez Valley. By the end of the project, 174 parcels had been transferred betweenMexico andTexas, each side receiving an equal area of land.[423][424] | too small to map |
| December 29, 1934 | Kingman Reef was placed under the jurisdiction of theUnited States Department of the Navy.[408] | no change to map |
| November 15, 1935 | TheInsular Government of the Philippine Islands wasdissolved and replaced with theCommonwealth of the Philippines.[425][426] | Pacific Ocean: |
| March 16, 1936 | The de jure overlap betweenMichigan andWisconsin wasresolved by theSupreme Court of the United States.[427] |  |
| May 13, 1936 | Baker Island,Howland Island, andJarvis Island were formally annexed and placed under the jurisdiction of theUnited States Department of the Interior,[428] ending theUnited Kingdom's claim to Jarvis Island.[293] | Pacific Ocean: |
| June 22, 1936 | TheUnited States Virgin Islands wereorganized into a civil territory.[429] | no change to map |
| August 6, 1936 | Canton Island,Enderbury Island, andMcKean Island were claimed by theUnited Kingdom.[293] | Pacific Ocean: |
| April 6, 1939 | The condominium of theCanton and Enderbury Islands was established with theUnited Kingdom.[430] | Pacific Ocean: |
| July 27, 1939 | Panama gained a sovereign corridor that was carved out of thePanama Canal Zone connectingColón with the rest ofPanama, along with a three-dimensional "tube" of sovereignty for a future crossing over an American highway. A corridor consisting of the road from the Canal Zone border toMadden Dam was annexed to the Canal Zone.[431] | Caribbean Sea: |
| August 16, 1939 | This is the earliest date so far discovered for when the United States began claimingFakaofo,Funafuti,Hull Island,Niulakita,Nukufetau, andNukulaelae.[432] | Pacific Ocean: |
| December 10, 1941 | GovernorGeorge McMillin surrenderedGuam to theJapanese military.[433] | Pacific Ocean: |
| December 23, 1941 | The garrison onWake Island surrendered to theJapanese military.[434] | Pacific Ocean: |
| March 26, 1942 | The government of thePhilippines evacuated from the territory in the face ofJapanese advance. Agovernment-in-exile would be established inWashington, D.C., on May 17, 1942. TheUnited States Army Forces in the Far East would surrender on April 9, 1942, following theBattle of Bataan, and the final military holdouts would surrender on May 6, 1942, following theBattle of Corregidor.[435] | Pacific Ocean: |
| October 14, 1943 | TheSecond Philippine Republic was established as apuppet state ofJapan.[435] | Pacific Ocean: |
| August 10, 1944 | Guam wascaptured fromJapan.[433] | Pacific Ocean: |
| August 17, 1945 | TheSecond Philippine Republic, in exile inTokyo since April 3, 1945, was dissolved. The process of re-establishing the Commonwealth government on Philippine soil had started on October 23, 1944.[435] | Pacific Ocean: |
| September 4, 1945 | TheJapanese garrison onWake Island surrendered to the United States.[434] | Pacific Ocean: |
| Date | Event | Change map |
|---|---|---|
| July 4, 1946 | TheCommonwealth of the Philippinesbecame independent as theRepublic of the Philippines.[436] | Pacific Ocean: |
| July 18, 1947 | TheUnited Nations entrusted theTrust Territory of the Pacific Islands to the United States.[7] | Pacific Ocean: |
| January 1, 1949 | TheTokelau Islands were incorporated intoNew Zealand, which inherited the claims onAtafu,Fakaofo, andNukunono.[437] | Pacific Ocean: |
| August 1, 1950 | Guam wasorganized into a civil territory.[438][439] | no change to map |
| August 3, 1950 | Kansas andMissouri exchanged small portions of land along theMissouri River, to accord with shifts in the river following a flood in 1944.[440] |  |
| May 6, 1954 | Alabama andFlorida defined their border around the mouth of thePerdido River.[441][442] | not mapped |
| April 11, 1955 | Panama's corridor connectingColón with the rest ofPanama was realigned within thePanama Canal Zone. Several three-dimensional "tubes" of sovereignty were also created, allowing Panamanian bridges to pass over rivers and a highway at several locations within the Canal Zone.[443][444] | too small to map |
| August 23, 1955 | Several border locations of thePanama Canal Zone were redefined. Punta Paitilla, the land held onTaboga Island, and the remaining American holdings inColón andPanama City were ceded toPanama.[364][445] | Caribbean Sea: |
| January 3, 1959 | TheAlaska Territory was admitted as the forty-ninth state,Alaska.[303] | Northwestern North America: |
| August 21, 1959 | Most ofHawaii Territory was admitted as the fiftieth state,Hawaii.Palmyra Atoll was excluded from statehood and remained a territory.[280][353] | Pacific Ocean: |
| August 25, 1961 | About 20 acres of land was transferred fromMinnesota toNorth Dakota nearFargo, North Dakota.[190][446] | too small to map |
| January 14, 1964 | TheChamizal, a tract of land betweenEl Paso, Texas, andCiudad Juárez, Chihuahua, was divided between the United States andMexico.[447] |  |
| August 4, 1965 | TheCook Islands became self-governing fromNew Zealand. It claimed the atolls ofManihiki,Penrhyn,Pukapuka, andRakahanga.[448] | Pacific Ocean: |
| December 30, 1966 | Land onDiego Garcia in theIndian Ocean was leased from theUnited Kingdom for use as a military base.[449] | no change to map |
| April 25, 1971 | The lease of theCorn Islands fromNicaragua was terminated.[387] | |
| September 1, 1972 | The United States recognized the sovereignty of Honduras over theSwan Islands.[387][450] | Caribbean Sea: |
| March 1, 1977 | The United Statesclaimed maritime borders west of theStrait of Juan de Fuca, within theDixon Entrance, and in theBeaufort Sea that conflicted with claims ofCanada.[451] | no change to map |
| May 26, 1977 | Several parcels wereexchanged betweenTexas andMexico along theRio Grande in areas nearPresidio andHidalgo, Texas,[452] including theHorcón Tract, on which the town ofRío Rico was located,[453] and Beaver Island nearRoma, Texas. In addition, Mexico ceded 823 acres (3.33 km2) to the U.S., while the U.S. ceded 2,177 acres (8.81 km2) to Mexico, primarily to straighten sections of the Rio Grande for flood control.[454] |  |
| December 16, 1977 | Atreaty defining the maritime border withCuba was signed; though it has never been ratified by theUnited States Senate, it is provisionally enforced by agreement renewed every two years.[378] | no change to map |
| October 1, 1978 | Tuvalu became independent from theUnited Kingdom. It claimed the atolls ofFunafuti,Nukufetau,Nukulaelae, andNiulakita.[455] | Pacific Ocean: |
| July 12, 1979 | TheRepublic of Kiribati became independent from theUnited Kingdom. It claimedBirnie Island,Canton Island,Caroline Island,Christmas Island,Enderbury Island,Flint Island,Gardner Island,Hull Island,Malden Island,McKean Island,Phoenix Island,Starbuck Island,Sydney Island, andVostok Island. This dissolved the condominium of theCanton and Enderbury Islands.[456] | Pacific Ocean: |
| October 1, 1979 | ThePanama Canal Zone wasceded toPanama. The United States and Panama continued to share operational control of the canal until December 31, 1999, when it would be fully turned over to Panama.[457] The United States retained control over several hundred specified areas to be turned over inpiecemeal fashion over the years. | Caribbean Sea: |
| November 24, 1980 | The maritime border between the United States andVenezuela was defined.[378][458] | no change to map |
| September 17, 1981 | The United States recognized the sovereignty ofColombia overRoncador Bank andSerrana Bank, and the claim onQuita Sueño Bank was abandoned by the United States, as it was no longer above the seas at high tide, and thus the government considered it unclaimable.[387][459] | Caribbean Sea: |
| September 3, 1983 | The United Statesrecognized the sovereignty of theNew Zealand territory ofTokelau overAtafu,Fakaofo, andNukunono, and defined the maritime border with Tokelau.[378][387][460] | Pacific Ocean: |
| September 8, 1983 | The United Statesrecognized the sovereignty of theCook Islands overManihiki,Penrhyn,Pukapuka, andRakahanga, and the maritime border with the Cook Islands was defined.[378][387][461] | Pacific Ocean: |
| September 23, 1983 | The United Statesrecognized the sovereignty ofKiribati overBirnie Island,Canton Island,Caroline Island,Christmas Island,Enderbury Island,Flint Island,Gardner Island,Hull Island,Malden Island,McKean Island,Phoenix Island,Starbuck Island,Sydney Island, andVostok Island.[387][462] The United States recognized the sovereignty ofTuvalu overFunafuti,Nukufetau,Nukulaelae, andNiulakita.[387][463] | Pacific Ocean: |
| October 12, 1984 | TheInternational Court of Justice made its judgment on where the maritime border should be in theGulf of Maine between the United States and Canada.[464][378] No land changed hands. The scope of the case did not include the sovereignty of Machias Seal Island, but the judgment enabled defining the extent of the disputed water area around that island (an area of 210 square nautical miles).[451] | no change to map |
| October 21, 1986 | The Marshall Islands District of theTrust Territory of the Pacific Islands became independent as theRepublic of the Marshall Islands.[465] The Marshall Islanders had claimedWake Island as part of their territory since at least 1973, and continued that after independence.[466] | Pacific Ocean: |
| November 3, 1986 | Most of theTrust Territory of the Pacific Islands was dissolved by theUnited Nations. The districts of Chuuk, Pohnpei, and Yap became independent as theFederated States of Micronesia. The Mariana Islands District, having already been taking moves towards integration with the United States, became a territory of the United States, theCommonwealth of the Northern Mariana Islands.[465] | Pacific Ocean: |
| June 1, 1990 | The maritime border between the United States and theSoviet Union was provisionally defined.[467][378] The two countries agreed on this date to abide by the terms of the treaty pending its ratification and entry into force,[468] but while it was ratified by the United States Senate on September 16, 1991,[469] it was never ratified by the Soviet Union or its successor state,Russia. | no change to map |
| October 1, 1994 | The remaining district of theTrust Territory of the Pacific Islands, the Palau District, became independent as theRepublic of Palau, dissolving the TTPI.[470] | Pacific Ocean: |
| June 1, 1995 | The maritime border between the United States and territories of theUnited Kingdom in theCaribbean Sea was defined.[378][471][472] | no change to map |
| January 16, 1997 | Navassa Island was transferred to theUnited States Department of the Interior.[473][474] | |
| November 13, 1997 | The maritime border between the United States andMexico was defined.[378][475] | |
| May 26, 1998 | TheSupreme Courtruled that extra land added toEllis Island since the original island was officially granted toNew York in an interstate compact withNew Jersey in 1834 belonged to New Jersey, because the island was within the territorial waters of New Jersey. The original natural boundary of Ellis Island remained an enclave of New York.[476] |  |
| December 31, 1999 | All formerPanama Canal Zone parcels not turned over since 1979, as well as all joint canal operations areas, were transferred toPanama. | too small to map |
| January 17, 2001 | The maritime border between the United States andMexico on the continental shelf in the western Gulf of Mexico beyond 200 nautical miles was defined.[378][477] | no change to map |
| November 24, 2009 | Six islands along theRio Grande were ceded fromTexas toMexico, and three islands and two bancos were ceded from Mexico to Texas. The transfer, which had been pending for 20 years, was the first application of Article III of the1970 Boundary Treaty.[378][418][478] | too small to map |
| September 23, 2014 | The maritime border between the United States andNiue wasdefined.[378][479] The treaty was signed on May 13, 1997, but it was not ratified by the United States until at least 2002, and theUnited Nations shows it as entering into force on this date.[480] | no change to map |
| January 1, 2017 | The border betweenNorth Carolina andSouth Carolina was clarified following years of surveys and negotiation, moving 19 homes across state lines.[481][482] | too small to map |
| November 30, 2020 | TheState of Rhode Island and Providence Plantations adopted a constitutional amendment, renaming itself the State of Rhode Island.[483][484] |  |
TheBanco Convention of 1905 between the United States andMexico allowed, in the event of sudden changes in the course of theRio Grande (as by flooding), for the border to be altered to follow the new course.[485] The sudden changes often createdbancos (land surrounded by bends in the river that became segregated from either country by a cutoff, often due to rapid accretion or avulsion of the alluvial channel), especially in the Lower Rio Grande Valley. When these bancos are created, theInternational Boundary and Water Commission investigates if land previously belonging to the United States orMexico is to be considered on the other side of the border.[486] In all cases of these adjustments along the Rio Grande under the 1905 convention, which occurred on 37 different dates from 1910 to 1976, the transferred land was minuscule (ranging from one to 646 acres) and uninhabited.[487][488][489]
... it was not until the year 1918, when the Governor of the Canal Zone addressed a communication to the Secretary of Foreign Affairs of the Republic of Panama stating that the United States Government would require 50.6 hectares [125 acres], that the exact area expropriated was definitely known.
A survey was made and a map drawn up showing the boundary lines of the Punta Paitilla military reservation. Various areas were determined with reference to high and low water and the old and new city boundary lines. Permanent monuments were set and a description written up by metes and bounds. A tracing was made of the map of the military reservation known as area 'A'.
Approvals: United States – May 26, 1977 [;] Mexico – May 26, 1977
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