Tetrarchy

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Roman imperial dynasties
The Tetrarchy
Portrait of the Four Tetrarchs, two porphyry sculptures looted from the Philadelphion of Constantinople after 1204, now standing at the southwest corner of St Mark's Basilica, Venice
Chronology
Diocletian as Augustus 284–286 — with Maximian 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 — with Severus II & Maximinus II as Caesares Severus II and Galerius as Augusti of West and East 306–307 — with Maximinus II and Constantine I as Caesares Maxentius & Maximian as usurpers in Italy and Africa 306–308 Galerius as Augustus 307–308 — with Maximinus II as Caesar — with Constantine I as self-proclaimed Augustus Licinius & Galerius as Augusti of West and East 308–311 — with Maximinus II and Constantine 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 — with Constantine I as self-proclaimed Augustus Constantine I & Licinius I as Augusti of West and East 313–324 — with Licinius II, Constantine II & Crispus as Caesares — with Valerius Valens as Augustus of the West 316–317 — with Martinian as Augustus of the West 324
Succession
Preceded by Crisis of the Third Century Followed by Constantinian dynasty

Politics of ancient Rome
Periods
Roman Kingdom 753–509 BC Roman Republic 509–27 BC Roman Empire 27 BC – AD 395 Principate 27 BC – AD 284 Dominate AD 284–641 Western AD 395–476 Eastern AD 395–1453 Timeline
Constitution
Kingdom Republic Sullan republic Empire Augustan reforms Late Empire
Political institutions
Imperium Collegiality Auctoritas Roman citizenship Cursus honorum
Assemblies
Centuriate Curiate Plebeian Tribal
Ordinary magistrates
Consul Praetor Quaestor Promagistrate Aedile Tribune Censor Governor
Extraordinary magistrates
Corrector Dictator Magister equitum Consular tribune Rex Triumviri Decemviri
Public law
Mos maiorum Ius Senatus consultum Quaestio perpetua Senatus consultum ultimum
Titles and honours
Emperor Legatus Dux Officium Praeses Praefectus Vicarius Vigintisexviri Triumvir monetalis Lictor Magister militum Imperator Princeps senatus Pontifex maximus Augustus Caesar Tetrarch
Other countries
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The Tetrarchy was the system instituted by Roman emperor Diocletian in 293 AD to govern the ancient Roman Empire by dividing it between two emperors, the augusti, and their junior colleagues and designated successors, the caesares.

Initially Diocletian chose Maximian as his caesar 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 the augustus was likened to Jupiter, while his caesar was akin to Jupiter's son Hercules. Galerius and Constantius were appointed caesares in March 293. Diocletian and Maximian retired on 1 May 305, raising Galerius and Constantius to the rank of augustus. Their places as caesares were in turn taken by Valerius Severus and Maximinus Daza.

The orderly system of two senior and two junior rulers endured until Constantius died in July 306, and his son Constantine was unilaterally acclaimed augustus and caesar by his father's army. Maximian's son Maxentius contested Severus' title, styled himself princeps invictus, and was appointed caesar by his retired father in 306. Severus surrendered to Maximian and Maxentius in 307. Maxentius and Constantine were both recognized as augusti by Maximian that same year. Galerius appointed Licinius augustus for the west in 308 and elevated Maximinus Daza to augustus in 310.

Constantine's victory over Maxentius at the Battle 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, and Licinius II – as caesares 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 as augustus after losing the Battle of Chrysopolis, leaving Constantine in control of the entire empire.

The Constantinian dynasty's emperors retained some aspects of collegiate rule; Constantine appointed his son Constantius II as another caesar in 324, followed by Constans in 333 and his nephew Dalmatius in 335, and the three surviving sons of Constantine in 337 were declared joint augusti together, and the concept of the division of the empire under multiple joint emperors endured until the Fall of the Western Roman Empire. In the Eastern Roman Empire, augusti and caesares continued to be appointed sporadically.