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 Western civilization before AD 500
The slow decline of the Western Empire occurred over a period of roughly three centuries, culminating in 476, when Romulus Augustus, the last Emperor of the Western Roman Empire, was deposed by Odoacer, a Germanic chieftain. Some modern historians question the significance of this date, and not simply because Julius Nepos, the legitimate emperor recognized by the East Roman Empire, continued to live in Salona, Dalmatia, until he was assassinated in 480. More importantly, the Ostrogoths who succeeded considered themselves upholders of the direct line of Roman traditions and, as the historian Edward Gibbon noted, the Eastern Roman Empire continued until the Fall of Constantinople on May 29, 1453.