Tetrarchy

Roman imperial dynasties
The Tetrarchy
Chronology
Diocletian as Augustus		284–286
-with Maximian as Augustus of the West		286–293
-with Galerius and Constantius Chlorus as Caesares		293–305
Maximian as the sole Caesar		285–286
-with Diocletian as Augustus of the East		286–305
-with Galerius and Constantius Chlorus as Caesares		293–305
Galerius and Constantius Chlorus as Caesares		293–305
Galerius and Constantius Chlorus as Augusti of East and West		305–306
-with Severus and Maximinus Daia as Caesares		305–306
Galerius and Severus as Augusti of East and West		306–307
-with Maximinus Daia and Constantine the Great as Caesares		306–307
Galerius and Maxentius as Augusti of East and West		307–308
-with Maximinus Daia and Constantine the Great as Caesares		307–308
Galerius and Licinius as Augusti of East and West		308–311
-with Maximinus Daia and Constantine the Great as Caesares		308–311
Maxentius as usurper in Rome (and Asia Minor 311–312)		308–312
Maximinus Daia and Licinius as Augusti of East and West		311–312
-with Constantine the Great as Caesar (self proclaimed Augustus)		311–312
Licinius and Constantine the Great as Augusti of East and West		312–324
-with Licinius II and Constantine II, Crispus as Caesares		317–324
Succession
Preceded by Crisis of the Third Century		Followed by Constantinian dynasty

The term tetrarchy (from the Greek: τετραρχία, tetrarchia, "leadership of four [people]") describes any form of government where power is divided among four individuals, but in modern usage usually refers to the system instituted by Roman Emperor Diocletian in 293, marking the end of the Crisis of the Third Century and the recovery of the Roman Empire. This tetrarchy lasted until c. 313, when conflict eliminated most of the claimants to power, leaving Constantine in control of the western half of the empire, and Licinius in control of the eastern half.

Although the term "tetrarch" was current in antiquity, it was never used of the imperial college under Diocletian. Instead, the term was used to describe independent portions of a kingdom that were ruled under separate leaders. The tetrarchy of Judaea, established after the death of Herod the Great, is the most famous example of the antique tetrarchy. The term was understood in the Latin world as well, where Pliny 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).

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 a college led by a single supreme leader. When later authors described the period, this is what they emphasized: Ammianus had Constantius II admonish Julian for disobedience by appealing to the example in submission set by Diocletian's lesser colleagues; Julian himself compared the Diocletianic tetrarchs to a chorus surrounding a leader, speaking in unison under his command. Only Lactantius, a contemporary of Diocletian and a deep ideological opponent of the Diocletianic state, referred to the tetrarchs as a simple multiplicity of rulers.

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