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.
Western Roman Empire
In 476, after the Battle of Ravenna, the Roman Army in the West suffered defeat at the hands of Odoacer and his Germanic "foederati". Odoacer forced the deposition of emperor Romulus Augustulus and became the first King of Italy. In 480, following the assassination of the previous Western emperor Julius Nepos, the Eastern emperor Zeno dissolved the Western court and proclaimed himself the sole emperor of the Roman Empire. The date of 476 was popularized by the 18th century British historian Edward Gibbon as a demarcating event for the end of the Western Empire and is sometimes used to mark the transition from Antiquity to the Middle Ages. Odoacer's Italy, and other barbarian kingdoms, would maintain a pretence of Roman continuity through the continued use of the old Roman administrative systems and nominal subservience to the Eastern Roman court.