The term Tetrarchy (Greek: "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 internecine conflict eliminated most of the claimants to power, leaving Constantine in the West and Licinius in the East.

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).

Constantine the Great (Latin: Flavius Valerius Aurelius Constantinus Augustus; c. 27 February 272 – 22 May 337), also known as Constantine I or Saint Constantine, was Roman Emperor from 306 to 337. Well known for being the first Roman emperor to convert to Christianity, Constantine and co-Emperor Licinius issued the Edict of Milan in 313, which proclaimed religious tolerance of all religions throughout the empire.

The foremost general of his time, Constantine defeated the emperors Maxentius and Licinius during civil wars. He also fought successfully against the Franks, Alamanni, Visigoths, and Sarmatians during his reign—even resettling parts of Dacia which had been abandoned during the previous century. Constantine built a new imperial residence in place of Byzantium, naming it New Rome. However, in Constantine's honour, people called it Constantinople, which would later be the capital of the Eastern Roman Empire for over one thousand years. Because of this, he is thought of as the founder of the Eastern Roman Empire.