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.
Ostrogothic Ravenna
In 476 the last emperor of the Western Roman Empire, Romulus Augustus, was forced to abdicate under invasions by Goths led by Odovacar. Upon the sacking of Rome by these Goths, Odovacar declared himself King of Italy and was able to conquer the peninsula as well as much of the "Dalmatian coast". Odovacar drew the ire of the imperial court at Constantinople as "a barbarian, a non-Roman, had deposed the Roman emperor in the original heartland of the empire. He had taken on the hated royal title and was getting ready to rule in the emperor’s place..." Odovacar, however, issued his coinage in the name of the emperor Nepos, whom Constantinople had never removed its recognition of when Romulus had assumed the imperial title therefore painting himself as merely overthrowing a previous usurper to the imperial throne.