TheTetrarchy was the system instituted byRoman emperorDiocletian in 293 AD to govern the ancient Roman Empire by dividing it between two emperors, theaugusti, and their junior colleagues and designated successors, thecaesares.

Roman imperial dynasties
The Tetrarchy
Portrait of the Four Tetrarchs, twoporphyry sculptures looted from thePhiladelphion of Constantinople after 1204, now standing at the southwest corner ofSt Mark's Basilica,Venice
Chronology
Diocletian as Augustus 284–286 — withMaximian as Caesar 285–286 Maximian &Diocletian as Augusti of the West and East 286–305 — with Constantius I & Galerius as Caesares 293–305 Constantius I &Galerius as Augusti of West and East 305–306 — withSeverus II &Maximinus II as Caesares Severus II andGalerius as Augusti of West and East 306–307 — withMaximinus II andConstantine I as Caesares Maxentius &Maximian as usurpers in Italy and Africa 306–308 Galerius as Augustus 307–308 — withMaximinus II as Caesar — withConstantine I as self-proclaimed Augustus Licinius &Galerius as Augusti of West and East 308–311 — withMaximinus II andConstantine I as Caesares Maxentius as usurper in Rome (and Asia Minor 311–312) 308–312 Licinius I &Maximinus II as Augusti of West and East 311–313 — withConstantine I as self-proclaimed Augustus Constantine I &Licinius I as Augusti of West and East 313–324 — withLicinius II,Constantine II &Crispus as Caesares — withValerius Valens as Augustus of the West 316–317 — withMartinian as Augustus of the West 324
Succession
Preceded by Crisis of the Third Century Followed by Constantinian dynasty

Initially Diocletian choseMaximian as hiscaesar in 285, raising him to co-augustus the following year; Maximian was to govern the western provinces and Diocletian would administer the eastern ones. The role of theaugustus was likened toJupiter, while hiscaesar was akin to Jupiter's sonHercules.Galerius andConstantius were appointedcaesares in March 293. Diocletian and Maximian retired on 1 May 305, raising Galerius and Constantius to the rank ofaugustus. Their places ascaesares were in turn taken byValerius Severus andMaximinus Daza.

The orderly system of two senior and two junior rulers endured until Constantius died in July 306, and his sonConstantine was unilaterally acclaimedaugustus andcaesar by his father's army. Maximian's sonMaxentius contested Severus' title, styled himselfprinceps invictus, and was appointedcaesar by his retired father in 306. Severus surrendered to Maximian and Maxentius in 307. Maxentius and Constantine were both recognized asaugusti by Maximian that same year. Galerius appointedLiciniusaugustus for the west in 308 and elevated Maximinus Daza toaugustus in 310.

Constantine's victory over Maxentius at theBattle of the Milvian Bridge in 312 left him in control of the western part of the empire, while Licinius was left in control of the east on the death of Maximinus Daza. Constantine and Licinius jointly recognized their sons –Crispus,Constantine II, andLicinius II – ascaesares in March 317. Ultimately the tetrarchic system lasted until c. 324, when mutually destructive civil wars eliminated most of the claimants to power:Licinius resigned asaugustus after losing theBattle of Chrysopolis, leaving Constantine in control of the entire empire.

TheConstantinian dynasty's emperors retained some aspects of collegiate rule; Constantine appointed his sonConstantius II as anothercaesar in 324, followed byConstans in 333 and his nephewDalmatius in 335, and the three surviving sons of Constantine in 337 were declared jointaugusti together, while the concept of the division of the empire under multiple joint emperors endured until theFall of the Western Roman Empire. In theEastern Roman Empire,augusti andcaesares continued to be appointed sporadically.

The termtetrarchy (from theGreek:τετραρχία,tetrarchia, "leadership of four [people]")^[a] describes any form of government where power is divided among four individuals.

Although the term "tetrarch" was current in antiquity, it was never used in the imperial college (as it's often called) under Diocletian. Instead, the term was used to describe independent portions of a kingdom that were ruled under separate leaders. Thetetrarchy of Judaea, established after the death ofHerod the Great, is the most famous example of the antique tetrarchy. The term was understood in the Latin world as well, wherePliny the Elder glossed it as follows: "each is the equivalent of a kingdom, and also part of one" (regnorum instar singulae et in regna contribuuntur).^[1]

As used by the ancients, the term describes not only different governments, but also a different system of government from the Diocletianic arrangements. The Judaean tetrarchy was a set of four independent and distinct states, where each tetrarch ruled a quarter of a kingdom as they saw fit; the Diocletianic tetrarchy was acollege led by a single supreme leader. When later authors described the period, this is what they emphasized:Ammianus had Constantius II admonishGallus for disobedience by appealing to the example in submission set by Diocletian's lesser colleagues; his successor Julian compared the Diocletianic tetrarchs to a chorus surrounding a leader, speaking in unison under his command.^[2] OnlyLactantius, a contemporary of Diocletian and a deep ideological opponent of the Diocletianic state, referred to the tetrarchs as a simple multiplicity of rulers.^[3]

Much modern scholarship was written without the term. AlthoughEdward Gibbon pioneered the description of the Diocletianic government as a "New Empire", he never used the term "tetrarchy"; neither didTheodor Mommsen. It did not appear in the literature until used in 1887 by schoolmaster Hermann Schiller in a two-volume handbook on the Roman Empire (Geschichte der Römischen Kaiserzeit), to wit: "die diokletianische Tetrarchie". Even so, the term did not catch on in the literature untilOtto Seeck used it in 1897.^[4]

The first phase, sometimes referred to as thediarchy ("rule of two"), involved the designation of the generalMaximian as co-emperor—firstly ascaesar (heir apparent) in 285, followed by his promotion toaugustus in 286.Diocletian took care of matters in the eastern regions of the empire while Maximian similarly took charge of the western regions. In 293, Diocletian thought that more focus was needed on both civic and military problems, so with Maximian's consent, he expanded the imperial college by appointing twocaesares (one responsible to eachaugustus)—Galerius andConstantius I.^[5]

In 305, the senior emperors jointly abdicated and retired, allowing Constantius and Galerius to be elevated in rank toaugustus. They in turn appointed two newcaesares—Severus II in the west under Constantius, andMaximinus in the east under Galerius—thereby creating the second Tetrarchy.

The four tetrarchs based themselves not at Rome but in other cities closer to the frontiers, mainly intended as headquarters for the defence of the empire against bordering rivals (notablySassanian Persia) andbarbarians (mainly Germanic, and an unending sequence of nomadic or displaced tribes from the eastern steppes) at theRhine andDanube. These centres are known as the tetrarchic capitals. Although Rome ceased to be an operational capital, Rome continued to be nominal capital of the entire Roman Empire, not reduced to the status of a province but under its own, unique Prefect of the city (praefectus urbi, later copied in Constantinople).

The four tetrarchic capitals were:

• Nicomedia in northwestern Asia Minor (modernİzmit in Turkey), a base for defence against invasion from the Balkans and Persia's Sassanids was the capital of Diocletian, the eastern (and most senior)augustus; in the final reorganisation by Constantine the Great, in 318, the equivalent of his domain, facing the most redoubtable foreign enemy,Sassanid Persia, became the praetorian prefecture Oriens, 'the East', the core of later Byzantium.
• Sirmium (modernSremska Mitrovica in theVojvodina region of modernSerbia, and nearBelgrade, on theDanube border) was the capital of Galerius, the easterncaesar; this was to become the Balkans-Danube prefecture Illyricum.
• Mediolanum (modernMilan, near the Alps) was the capital of Maximian, the westernaugustus; his domain became "Italia et Africa", with only a short exterior border.
• Augusta Treverorum (modernTrier, in Germany) was the capital of Constantius, the westerncaesar, near the strategic Rhine border; it had been the capital of Gallic emperorTetricus I. This quarter became the prefecture Galliae.

Aquileia, a port on the Adriatic coast, andEboracum (modernYork, in northern England near the Celtic tribes of modern Scotland and Ireland), were also significant centres for Maximian and Constantius respectively.

In terms of regional jurisdiction there was no precise division among the four tetrarchs, and this period did not see the Roman state actually split up into four distinct sub-empires. Each emperor had his zone of influence within the Roman Empire, but little more, mainly high command in a 'war theater'. Each tetrarch was himself often in the field, while delegating most of the administration to the hierarchic bureaucracy headed by his respectivepraetorian prefect, each supervising severalvicarii, the governors-general in charge of another, lasting new administrative level, the civildiocese. For a listing of the provinces, now known aseparchy, within each quarter (known as a praetorian prefecture), seeRoman province.

In the West, theaugustus Maximian controlled the provinces west of theAdriatic Sea and theSyrtis, and within that region hiscaesar, Constantius, controlled Gaul and Britain. In the East, the arrangements between theaugustus Diocletian and hiscaesar, Galerius, were much more flexible.

Although power was shared in the tetrarchic system, the public image of the four members of the imperial college was carefully managed to give the appearance of a united empire (patrimonium indivisum). This was especially important afterthe numerous civil wars of the 3rd century.

The tetrarchs appeared identical in all official portraits. Coinage dating from the tetrarchic period depicts every emperor with identical features—only the inscriptions on the coins indicate which one of the four emperors is being shown. The Byzantine sculpturePortrait of the Four Tetrarchs shows the tetrarchs again with identical features and wearing the same military costume.

One of the greatest problems facing emperors in the Third Century Crisis was that they were only ever able to personally command troops on one front at any one time. WhileAurelian andProbus were prepared to accompany their armies thousands of miles between war regions, this was not an ideal solution. Furthermore, it was risky for an emperor to delegate power in his absence to a subordinate general, who might win a victory and then be proclaimed as a rival emperor himself by his troops (which often happened). All members of the imperial college, on the other hand, were of essentially equal rank, despite two being senior emperors and two being junior; their functions and authorities were also equal.

Under the Tetrarchy a number of important military victories were secured. Both the dyarchic and the tetrarchic system ensured that an emperor was near to every crisis area to personally direct and remain in control of campaigns simultaneously on more than just one front. After suffering a defeat by thePersians in 296, Galerius crushedNarseh in 298—reversing a series of Roman defeats throughout the century—capturing members of the imperial household and a substantial amount of booty and gaining a highly favourable peace treaty, which secured peace between the two powers for a generation. Similarly, Constantius defeated the British usurperAllectus, Maximian pacified the Gauls, and Diocletian crushed the revolt ofDomitianus inEgypt.

When in 305 the 20-year term of Diocletian and Maximian ended, both abdicated. Theircaesares, Galerius and Constantius Chlorus, were both raised to the rank ofaugustus, and two newcaesares were appointed:Maximinus Daza (caesar to Galerius) andValerius Severus (caesar to Constantius). These four formed the second tetrarchy.

However, the system broke down very quickly thereafter. When Constantius died in 306,Constantine, Constantius' son, was proclaimedaugustus by his father's troops; however, Galerius instead chose to promote Severus toaugustus while granting Constantine the position of caesar to Severus. At the same time,Maxentius, the son of Maximian, resented being left out of the new arrangements, so he rebelled against and defeated Severus before forcing him to abdicate and then arranging his murder in 307. Maxentius and Maximian both then declared themselvesaugusti. By 308 there were therefore no fewer than four claimants to the rank ofaugustus (Galerius, Constantine, Maximian and Maxentius), and only one to that ofcaesar (Maximinus Daza).

In 308 Galerius, together with the retired emperor Diocletian and the supposedly retired Maximian, called an imperial "conference" atCarnuntum on the River Danube. The council agreed thatLicinius would becomeaugustus in the West, with Constantine as hiscaesar. In the East, Galerius remainedaugustus and Maximinus remained hiscaesar. Maximian was to retire, and Maxentius was declared a usurper. This agreement proved disastrous: by 308 Maxentius had becomede facto ruler of Italy and Africa even without any imperial status, and neither Constantine nor Maximinus—who had both beencaesares since 306 and 305 respectively—were prepared to tolerate the promotion of theaugustus Licinius as their superior.

After an abortive attempt to placate both Constantine and Maximinus with the meaningless titlefilius augusti ("son of theaugustus", essentially an alternative title forcaesar), they both had to be recognised as Augusti in 309. However, four full Augusti all at odds with each other did not bode well for the tetrarchic system.

Between 309 and 313 most of the claimants to the imperial office died or were killed in various civil wars. Constantine forced Maximian's suicide in 310. Galerius died naturally in 311. Maxentius was defeated by Constantine at theBattle of the Milvian Bridge in 312 and subsequently killed. Maximinus committed suicide atTarsus in 313 after being defeated in battle by Licinius.

By 313, therefore, there remained only two rulers: Constantine in the West and Licinius in the East. The tetrarchic system was at an end, although it took until 324 for Constantine to finally defeat Licinius, reunite the two halves of the Roman Empire and declare himself soleaugustus.^[6]

Twocaesares are appointed in 293, thus starting the Tetrarchy.

After the retirement of the two augusti both previouscaesares succeeded them, and two newcaesares were appointed.

After the sudden death ofConstantius Chlorus (who died of natural causes), thecaesarFlavius Severus succeeded him asaugustus. However, Constantius' troops immediately proclaimed Constantine, Constantius' son, as their newaugustus. Galerius accepted Constantine as part of the imperial college, but only ascaesar. On 28 October 306, Maximian's sonMaxentius proclaimed himself emperor in Rome. Maximian also proclaimed himself emperor, ruling jointly with his son. Despite being accepted by theRoman Senate, they were not recognized by the other emperors.^[8]

Severus was taken hostage by Maximian in April 307, but Galerius still acknowledged him as the official emperor of the west. Constantine was denied the promotion toaugustus even after Severus' death in September, as Galerius had decided to exclude him from the system altogether. Maximian acknowledge Constantine's status asaugustus, but this meant nothing given that he himself was declared an usurper. Galerius and Maximinus thus remained as the only "legimitate" members of the imperial college.^[8]

At the council of Carnutum, Diocletian decides that Licinius will be the newaugustus of the west (although his western domains only consist of theDiocese of Pannonia). Constantine was given back the title ofcaesar, which he continued to unacknowledge.^[8]

Maximinus was proclaimedaugustus by his troops in about May 310. Galerius reluctantly agreed to recognize both Maximinus and Constantine asaugusti, thus breaking the Diocletian's tetrarchic system.^[8]

After the death of Galerius' (who died of natural causes), Licinius acquires parts of his domains, thus ruling over territories both in the East and West.

Licinius eventually fights and defeats Maximinus, gaining all eastern territories. He then makes peace with Constantine, who remains as the emperor of the West. This joint rule lasted until 316, when Licinius rejected Constantine's election ofBassianus ascaesar. In the ensuing war, bothaugusti appointed their own sons ascaesares, restoring a dynastic system. Licinius appointedValens andMartinian asaugustus in 316 and 324 respectively (literary sources refer to them ascaesar, but coins bear the titleaugustus); almost nothing is known about them.

Although the tetrarchic system as such only lasted until 313, many aspects of it survived. The fourfold regional division of the empire continued in the form ofPraetorian prefectures, each of which was overseen by apraetorian prefect and subdivided into administrativedioceses, and often reappeared in the title of the military supra-provincial command assigned to amagister militum.

The pre-existing notion ofconsortium imperii, the sharing of imperial power, and the notion that an associate to the throne was the designated successor (possibly conflicting with the notion of hereditary claim by birth or adoption), was to reappear repeatedly.

The idea of the two halves, the east and the west, re-emerged and eventually resulted in the permanent de facto division into two separate Roman empires after the death ofTheodosius I; though, importantly, the Empire was never formally divided. The emperors of the eastern and western halves legally ruled as one imperial college until theFall of the Western Roman Empire left Byzantium, the "second Rome", as the sole direct heir.

• Tetrarchies in the ancient world existed in bothThessaly (in northern Greece) andGalatia (in centralAsia Minor; includingLycaonia) as well as among the BritishCantiaci.
• The constellation of Jewish principalities in theHerodian kingdom ofJudea was known as a tetrarchy; seeTetrarchy (Judea).
• In the novelThe Lion, the Witch and the Wardrobe, thePevensie siblings ruleNarnia as a tetrarchy of two kings and two queens.

• Notitia dignitatum, a later document from the imperial chancery
• Problem of two emperors – Problem arising when multiple people claim the title of emperor

• Barnes, Timothy D. (1984).Constantine and Eusebius. Harvard University Press.ISBN 0674165314.
• Barnes, Timothy D. (1982).The New Empire of Diocletian and Constantine. Cambridge, MA: Harvard University Press.doi:10.4159/harvard.9780674280670.ISBN 0-674-28066-0.
• Bowman, Alan (1939).The Cambridge Ancient History Volume 12, The Crisis of Empire, AD 193–337. Cambridge University Press.ISBN 9780521044943.
• Corcoran, Simon (2000).The Empire of the Tetrarchs, Imperial Pronouncements and Government AD 284–324. Oxford University Press.ISBN 019815304X.
• Kolb, Frank (2011).Diocletian und die Erste Tetrarchie: Improvisation oder Experiment in der Organisation monarchischer Herrschaft? (in German). Berlin: de Gruyter.ISBN 978-3110846508.
• Kuhoff, Wolfgang (2001).Diokletian und die Epoche der Tetrarchie: Das römische Reich zwischen Krisenbewältigung und Neuaufbau (284–313 n. Chr.). Frankfurt am Main: Lang.ISBN 978-3631367926.
• Leadbetter, William Lewis (2009).Galerius and the Will of Diocletian. London; New York: Routledge.ISBN 978-1135261320.
• Rees, Roger (2004).Diocletian and the Tetrarchy. Edinburgh: Edinburgh University Press.ISBN 978-0748616602.

• A detailed chronology of the tetrarchy from Diocletian to Constantine
• A chart showing the tetrarchy from Diocletian to Constantine