Tetrarchy

The Tetrarchy was the system instituted by Roman emperor Diocletian to govern the ancient Roman Empire by dividing it between two emperors, the augusti, and their junior colleagues and designated successors, the caesares. It was kept in place between AD 293 and 313.

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, while 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.

The term tetrarchy (from the Greek: τετραρχία, tetrarchia, "leadership of four [people]") 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. 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 Gallus 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. 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.