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
After 395, the emperors in the Western Roman Empire were usually figureheads, while the actual rulers were military strongmen who took the title of "magister militum", "patrician" or both—Stilicho from 395 to 408, Constantius from about 411 to 421, Aëtius from 433 to 454 and Ricimer from about 457 to 472. The year 476 is generally accepted as the formal end of the Western Roman Empire. That year, Orestes, having stolen power from the emperor Julius Nepos the year before, refused the request of Germanic mercenaries in his service for lands in Italy. The dissatisfied mercenaries, including the Heruli, revolted. The revolt was led by the Germanic chieftain Odoacer. Odoacer and his men captured and executed Orestes; weeks later they captured Ravenna and deposed Orestes' usurper son, Romulus Augustus. This event has been traditionally considered the fall of the Roman Empire in the West. Odoacer quickly conquered the remaining provinces of Italy.