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Adeclaration of war is a formal declaration issued by a national government indicating that a state of war exists between that nation and another. A document by theFederation of American Scientists gives an extensive listing and summary of statutes which are automatically engaged upon theUnited States declaring war.[1]
For the United States,Article One, Section Eight of theConstitution says "Congress shall have power to ... declare War." However, that passage provides no specific format for what form legislation must have in order to be considered a "declaration of war" nor does the Constitution itself use this term. In the courts, theUnited States Court of Appeals for the First Circuit, inDoe v. Bush, said: "[T]he text of theOctober Resolution itself spells out justifications for a war and frames itself as an 'authorization' of such a war",[2] in effect saying that an authorization suffices for declaration and that what some may view as a formal congressional "Declaration of War" was not required by the Constitution.
The last time the United States formally declared war, using specific terminology, on any nation was in 1942, when war was declared against Axis-alignedHungary,Bulgaria, andRomania, because PresidentFranklin Roosevelt thought it was improper to engage in hostilities against a country without a formal declaration of war. Since then, every American president has used military force without a declaration of war.[3]
This article will use the term "formal declaration of war" to mean congressional legislation that uses the phrase "declaration of war" in the title. Elsewhere, this article will use the terms "authorized by Congress," "funded by Congress" or "undeclared war" to describe other such conflicts.
The United States has formally declared war against foreign nations in five separate wars, each upon prior request by the president of the United States. Four of those five declarations came after hostilities had begun.[4] James Madison reported that in theFederal Convention of 1787, the phrase "make war" was changed to "declare war" in order to leave to the executive the power to repel sudden attacks but not to commence war without the explicit approval of Congress.[5] Debate continues as to the legal extent of the president's authority in this regard.
Public opposition to American involvement in foreign wars, particularly during the 1930s, was expressed as support for aConstitutional Amendment that would require anational referendum on a declaration of war.[6] Several constitutional amendments, such as theLudlow Amendment, have been proposed that would require a national referendum on a declaration of war.
After Congress repealed theGulf of Tonkin Resolution in January 1971 and PresidentRichard Nixon continued to wage war in Vietnam, Congress passed theWar Powers Resolution (Pub. L. 93–148) over the veto of Nixon in an attempt to rein in some of the president's claimed powers. The War Powers Resolution proscribes the only power of the president to wage war which is recognized by Congress.[7]
The United States has formally declared war in five separate conflicts, issuing declarations against ten different foreign nations. The only country to have been the subject of multiple U.S. war declarations is Germany, which the United States formally declared war against twice, once inWorld War I and again inWorld War II. A case could also be made for Hungary, as it was asuccessor state toAustria-Hungary, against which the U.S. declared war in World War I.
Under theUnited States Constitution,Congress holds the sole authority to issue formal war declarations. Since the nation's founding, Congress has exercised this power on a limited number of occasions.
The first formal declaration occurred onJune 18, 1812, when the United States declared war against theUnited Kingdom, citing Britishimpressment of American sailors and violations of U.S. neutrality during theNapoleonic Wars. This conflict, known as theWar of 1812, concluded with theTreaty of Ghent in 1814.[8]
OnMay 13, 1846, Congress declared war onMexico at the request of PresidentJames K. Polk, following territorial disputes and military clashes in Texas. TheMexican–American War resulted in theTreaty of Guadalupe Hidalgo (1848), which expanded U.S. territory to include present-dayCalifornia,Arizona, and other southwestern states.[9]
TheSpanish–American War marked another instance of a formal declaration of war. The conflict was precipitated by the sinking of theUSSMaine inHavana Harbor and growing tensions over Spanish colonial rule inCuba. The war ended later that year with theTreaty of Paris (1898), which granted the United States control over former Spanish territories, includingPuerto Rico,Guam, and thePhilippines.[10]
During World War I, the United States declared war onGermany onApril 6, 1917, citingunrestricted submarine warfare and theZimmermann telegram, in which Germany attempted to persuade Mexico to join the war against the United States. Later that year, onDecember 7, 1917, Congress also declared war onAustria–Hungary, a German ally. The conflict formally ended for the United States with the signing of separate peace treaties with Germany,Austria, andHungary in 1921.[11]
World War II saw the most formal war declarations by the United States. Following theattack on Pearl Harbor on December 7, 1941, Congress declared war onJapan the next day, with near-unanimous approval. In response,Nazi Germany andItaly, led byAdolf Hitler andBenito Mussolini, declared war on the United States onDecember 11, prompting reciprocal declarations by Congress against both nations. In 1942, the United States further declared war onBulgaria,Hungary, andRomania, allAxis-aligned nations.[12] The war concluded in 1945 with theunconditional surrender ofGermany andJapan, followed by formal peace treaties in the years after.[13]
Each of these declarations was issued through a Congressional resolution, signed into law by the president, signifying the commencement of hostilities under the authority of the U.S. government. Since World War II, the United States has engaged in numerous military conflicts, but none have been accompanied by a formal declaration of war. Instead, Congress has authorized the use of military force through resolutions, such as those inKorea,Vietnam, and thewar on terror, including conflicts inAfghanistan andIraq.
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In other instances, the United States has engaged in extended military combat that was authorized by Congress.
| War or conflict | Opponent(s) | Initial authorization | Votes | President | Details of Authorization | Result | |
|---|---|---|---|---|---|---|---|
| Senate | House | ||||||
| Quasi-War |  France | An Act further to protect the commerce of the United States July 9, 1798 | 18–4 | John Adams | Adams requested legislation allowing theUnited States Navy to defend American shipping after repeated attacks by theFrench Navy during theFrench Revolutionary Wars.[16] | Treaty of Mortefontaine | |
| First Barbary War |  Morocco | "An Act for the Protection of the Commerce and Seamen of the United States, Against the Tripolitan Cruisers", 2 Stat. 129, February 6, 1802[17] | Thomas Jefferson | President Jefferson requested legislation allowing the U. S. Navy to defend shipping in theMediterranean Sea from Tripolitanian vessels.[18] | War ended 1805 | ||
| Second Barbary War |  Algiers | "An Act for the protection of the commerce of the United States against the Algerine cruisers", 3 Stat. 230, May 10, 1815[19] | James Madison | Madison requested a declaration of war against Algiers citing attacks on U.S. shipping in the Mediterranean. Congress rejected the request for a formal war declaration but ratified legislation allowing the U.S. Navy to defend U.S. commerce.[18] | War ended 1816 | ||
| Enforcing 1808 slave trade ban; naval squadron sent to African waters to apprehend illegal slave traders | Slave traders | "Act in addition to the acts prohibiting the Slave Trade", 3 Stat. 532, 1819 | James Monroe | 1822 first African-American settlement founded in Liberia, 1823 U.S. Navy stops anti-trafficking patrols | |||
| Suppression of Piracy | Pirates | 1819 | The United States Congress passed legislation allowing the United States Navy to suppress piracy in response to the rise in piracy inLatin America and the Caribbean after theSpanish American wars of independence, which was later permanently codified asTitle 33 of the United States Code.[16] | ||||
| Redress for attack onU.S. Navy'sUSS Water Witch |  Paraguay | 1858[20] | James Buchanan | ||||
| Mexican Revolution |  Mexico | H.J.R. 251, 38 Stat. 770 April 22, 1914[21] | 337–37 | Woodrow Wilson | Force withdrawn after six months. However, the Joint Resolution was likely used to authorize thePancho Villa Expedition. In the Senate, "when word reached the Senate that the invasion had gone forward before the use-of-force resolution had been approved, Republicans reacted angrily" saying it was a violation of the Constitution, but eventually after the action had already started, a resolution was passed after the action to "justify" it since Senators did not think it was a declaration of war.[22][23] | ||
| Russian Civil War |
| 1918[24] | |||||
| Lebanon crisis of 1958 |  Lebanese Opposition | H.J. Res. 117, Public Law 85-7, Joint Resolution "To promote peace and stability in the Middle East", March 9, 1957[25] | 72–19 | 355–61 | Dwight D. Eisenhower | Eisenhower requested legislation allowing U.S. economic and military assistance to theMiddle East during theCold War, including the ability to deploy the military in response to threatened Communist takeovers.[26] | U.S. forces withdrawn, October 25, 1958 |
| Vietnam War | Gulf of Tonkin Resolution, August 7, 1964 | 88–2 | 416–0 | Lyndon B. Johnson | Johnson requested authorization for a military deployment to defendSouth Vietnam andU.S. military forces already stationed there from underSEATO collective security obligations, citing allegedVietnam People's Navy attacks on United States Navy warcraft including theUSS Maddox in theGulf of Tonkin incident. Congress responded with theGulf of Tonkin Resolution.[27] | U.S. forces withdrawn under terms of theParis Peace Accords signed January 27, 1973 | |
| Multinational Force in Lebanon |  Shia militias,Druze militias, Syria | S.J.Res. 159 Pub. L. 98–119 September 29, 1983 | 54–46 | 253–156 | Ronald W. Reagan | Reagan announced the deployment of a smallUnited States Marine Corps contingent of forces for peacekeeping in the Lebanese Civil War, claiming they would supervise thePLO withdrawal from Beirut and provide law enforcement, but not participate in direct combat. After Congress invoked theWar Powers Resolution, it and the Reagan administration negotiated a resolution allowing the marines to remain in Lebanon for 18 months.[28] | |
| Persian Gulf War |  Ba'athist Iraq | H.J.Res. 77 January 12, 1991. | 52–47 | 250–183 | George H.W. Bush | Bush announced the deployment of 330,000 soldiers to Saudi Arabia in response to theIraqi invasion of Kuwait and began diplomatic overtures to form aninternational coalition to defend theArab states of the Persian Gulf. Congress approved an Authorization for Use of Military Force againstBa'athist Iraq to liberate Kuwait underUnited Nations Security Council Resolution 678.[29] | TheUnited Nations Security Council drew up terms for the cease-fire, April 3, 1991. Theadministration of George W. Bush later argued that the AUMF never expired during thebuild-up to the Iraq War. Repealed under theNational Defense Authorization Act for Fiscal Year 2026.[30][31] |
| War on terror |  Afghanistan
| S.J. Res. 23 September 14, 2001 | 98–0 | 420–1 | George W. Bush | Bush successfully requested a congressional authorizing the president of the United States to use military force against "those nations, organizations, or persons he determines planned, authorized, committed, or aided theterrorist attacks that occurred on September 11, 2001" as well as governments which sheltered them such as theFirst Islamic Emirate of Afghanistan. It also allowed the president to use force to prevent future acts ofterrorism. Since then the authorization has been invoked in conflicts in 22 countries against the original perpetrator of 9/11al-Qaeda as well as other organizations such asAl-Shabaab, theTaliban, and theIslamic State. The authorization is also notable in that it delegated war powers related to terrorism from Congress to the president, and allowed the United States to make war against individuals and organizations in addition to sovereign states. The Supreme Court ruled inHamdi v. Rumsfeld that the authorization enabled the president to detain individuals, including U.S. citizens, asenemy combatants, although it granted detainees to challenge this status in U.S. courts and further ruled inHamdan v. Rumsfeld that they were protected bylaws of war such as theGeneva Conventions and theUniform Code of Military Justice.[32][33][34][35][36] | The Global War on Terror is ongoing. TheWar in Afghanistan (2001–2021), that was carried out by the United States under the Global War on Terror's general authorization for use of military force, came to an end on August 30, 2021 with the total withdrawal of the American Forces from Afghanistan under the terms of theDoha Peace Agreement signed on February 29, 2020. The U.S. disengagement from Afghanistan resulted in theFall of Kabul to the Taliban on August 15, 2021 and in a broad re-establishment of thestatus quo ante bellum. The U.S. backedIslamic Republic of Afghanistan collapsed even before the completion of the American withdrawal, and the Taliban victory led to the restoration of theIslamic Emirate of Afghanistan. Other U.S. military campaigns that are legally based on the Global War on Terror's general authorization for use of military force include the ongoingAmerican-led intervention in the Syrian civil war that was initiated on September 22, 2014 under PresidentBarack Obama's administration. In spite of a significant drawdown of U.S. ground forces in Syria at the direction of PresidentDonald Trump in 2019, the United States retains a residual presence of about 600 military personnel in Syria, and continues to conduct airstrikes against Iranian-supported militias as of 2021. TheUnited States House of Representatives voted to repeal the 2001 AUMF in 2021.[37] |
| Iraq War[38] | Ba'athist Iraq | H.J. Res. 114, March 3, 2003 | 77–23 | 296–132 | During theIraq disarmament crisis Bush successfully requested an authorization of military force against Iraq alleging violations ofUnited Nations Security Council resolutions including the ceasefire with Kuwait, illegalweapons of mass destruction programs, and the sheltering of al-Qaeda members in the country. The Bush administration also claimed that the conflict was sanctioned by the 1991 AUMF against Iraq and by theIraq Liberation Act of 1998 designating the overthrow ofSaddam Hussein's dictatorship as a goal ofU.S. foreign policy.[39][40] | Ba'athist Iraqi government abolished April 2003, Saddam Hussein executed. War ended December 15, 2011. Destabilization of Iraq and emergence ofISIL (ISIS) in Iraq region 2014–2017.[41]
| |
In many instances, the United States has engaged in extended military engagements that were authorized byUnited Nations Security Council Resolutions and funded by appropriations from Congress.[42]
| Military engagement | Opponent(s) | Initial authorization | President | Result |
|---|---|---|---|---|
| Korean War |  China North Korea Soviet Union | UNSCR 84, 1950 | Harry S. Truman | Korean Armistice Agreement,[43] 1953 |
| Multinational Force in Lebanon | Shia militias,Druze militias,Syria | UNSCR 425, 1978 UNSCR 426, 1978 | Jimmy Carter,Ronald Reagan | U.S. forces withdrew in 1984 |
| Persian Gulf War |  Iraq | UNSCR 678, 1990 | George H. W. Bush | UNSCR 689, 1991 |
| Bosnian War |  Republika Srpska | UNSCR 770, 1992 UNSCR 776, 1992 UNSCR 836, 1993 | Bill Clinton | Reflagged asIFOR in 1995, Reflagged asSFOR in 1996, Completed in 2004 |
| Second Liberian Civil War | N/A | UNSCR 1497, 2003 | George W. Bush | U.S. forces are withdrawn in 2003 after theUNMIL is established. |
| Haitian coup d'état | UNSCR 1529, 2004 UNSCR 1542, 2004 | 2004 | ||
| First Libyan Civil War |  Libya | UNSCR 1973, 2011 | Barack Obama | Debellation of theLibyan Arab Jamahiriya, October 31, 2011 |
On at least 125 occasions, the president has acted without prior express military authorization from Congress.[44] These include instances in which the United States fought in thePhilippine–American War from 1898 to 1903, inNicaragua in 1927, as well as theNATO bombing campaign ofYugoslavia in 1999, and the2018 missile strikes onSyria.
TheIndian Wars comprise at least 28 conflicts and engagements. These localized conflicts, withNative Americans, began with European colonists coming to North America, long before the establishment of the United States. For the purpose of this discussion, the Indian Wars are defined as conflicts with the United States of America. They begin as one front in theAmerican Revolutionary War in 1775 and had concluded by 1918. TheUnited States Army still maintains acampaign streamer forPine Ridge 1890–1891 despite opposition from certain Native American groups.[45]
TheAmerican Civil War was not an international conflict under the laws of war, because theConfederate States of America (CSA) was not a government that had been granted full diplomatic recognition as asovereign nation by other sovereign states[46][47] or by the government of the United States.[48]
The US joinedthe Israeli war on Iran on the night of June 22, 2025 without a declaration of war or congressional approval.
On March 21, 2011, a number of lawmakers expressed concern that the decision of President Barack Obama to order the U.S. military to join in attacks ofLibyan air defenses and government forces exceeded his constitutional authority because the decision to authorize the attack was made without congressional permission.[49]
confederacy recognition.
The Confederate States did not constitute a new state and they were not independent, hence they were not entitled to treatment by the United States as a foreign state.
