The transition from a single united empire to the later divided Western and Eastern empires was a gradual transformation. In July 285, Diocletian defeated rival Emperor Carinus and briefly became sole emperor of the Roman Empire.

Diocletian saw that the vast Roman Empire was ungovernable by a single emperor in the face of internal pressures and military threats on two fronts. He therefore split the Empire in half along a northwest axis just east of Italy, and created two equal Emperors to rule under the title of Augustus. Diocletian himself was the Augustus of the eastern half, and he made his long-time friend Maximian Augustus of the western half. In doing so, he effectively created what would become the Western Roman Empire and the Eastern Roman Empire.

In 293 authority was further divided, as each Augustus took a junior Emperor called a Caesar to aid him in administrative matters, and to provide a line of succession; Galerius became Caesar under Diocletian and Constantius Chlorus Caesar under Maximian. This constituted what is called the Tetrarchy (in Greek: "leadership of four") by modern scholars. After Rome had been plagued by bloody disputes about the supreme authority, this finally formalised a peaceful succession of the emperor: in each half a Caesar would rise up to replace the Augustus and select a new Caesar. On 1 May 305, Diocletian and Maximian abdicated in favor of their Caesars. Galerius named the two new Caesars: his nephew Maximinus for himself, and Flavius Valerius Severus for Constantius. The arrangement worked well under Diocletian and Maximian and shortly thereafter. The internal tensions within the Roman government were less acute than they had been. In The Decline and Fall of the Roman Empire, Edward Gibbon notes that this arrangement worked well because of the affinity the four rulers had for each other. Gibbon says that this arrangement has been compared to a "chorus of music". With the withdrawal of Diocletian and Maximian, this harmony disappeared.

After an initial period of tolerance, Diocletian, who was a fervent pagan and was worried about the ever-increasing numbers of Christians in the Empire, persecuted them with zeal unknown since the time of Nero; this was to be one of the greatest persecutions the Christians endured in history.