History of the East–West Schism
Disunion in the Roman Empire further contributed to disunion in the Church. The Emperor Diocletian famously divided the administration of the eastern and western portions of the Empire in the early 4th century, though subsequent leaders (including Constantine) aspired to and sometimes gained control of both regions. Theodosius the Great, who established Nicene Christianity as the official religion of the Roman Empire (see Edict of Thessalonica), was the last Emperor to rule over a united Roman Empire. Following his death in 395, the division into western and eastern halves, each for a few decades still under its own Emperor, was never reunited.
History of the Roman Empire
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's reign stabilized the empire and marked the end of the Crisis of the Third Century. Diocletian appointed a co-emperor in 286 and delegated further with two junior-emperors.