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
The Western Roman Empire began to disintegrate in the early 5th century as Germanic migrations and invasions overwhelmed the capacity of the Empire to assimilate the migrants and fight off the invaders. The Romans were successful in fighting off all invaders, most famously Attila the Hun, though the Empire had assimilated so many Germanic peoples of dubious loyalty to Rome that the Empire started to dismember itself. Most chronologies place the end of the Western Roman empire in 476, when Romulus Augustulus was forced to abdicate to the Germanic warlord Odoacer. By placing himself under the rule of the Eastern Emperor, rather than naming himself Emperor as other Germanic chiefs had done, Odoacer ended the Western Empire by ending the line of Western Emperors. The eastern Empire exercised diminishing control over the west over the course of the next century. The empire in the East—known today as the Byzantine Empire, but referred to in its time as the "Roman Empire" or by various other names—ended in 1453 with the death of Constantine XI and the fall of Constantinople to the Ottoman Turks.