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.
Fall of Ravenna
The Fall of Ravenna, capital of the Western Roman Empire, occurred in early September 476 after a minor confrontation between the Heruli under their King Odoacer and the remnants of the Western Roman Army in Italy. The Roman Empire had been in relative decline since the beginning of the barbarian invasions and Rome, the symbolical heart and largest city of the Western Empire, was sacked in 410 by the Visigoths and in 455 by the Vandals. By 476 the Roman Emperor was little more than a warlord having very little de facto control of any territory outside of Italy. The last Roman emperor, Romulus Augustulus, was not even recognized as a legitimate ruler outside of Italy; the Eastern Roman Empire recognized Julius Nepos as the true Western Roman Emperor.