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
As a matter of historical convention, the late Roman Empire emerged from the Roman Principate (the early Roman Empire), with the accession of Diocletian in 284, his reign marking the beginning of the Dominate, or Late Roman Empire. It may begin with the commencement of the reign of Diocletian in AD 284, following the Third Century Crisis of AD 235–284, and to end in the west with the collapse of the Western Empire in AD 476, while in the east its end is disputed, as either occurring at the close of the reign of Justinian I (AD 565) or of Heraclius (AD 641).