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.
Historiography of the fall of the Western Roman Empire
In June 474, Julius Nepos became Western Emperor but in the next year the "magister militum" Orestes revolted and made his son Romulus Augustus emperor. Romulus, however, was not recognized by the Eastern Emperor Zeno and so was technically an usurper, Nepos still being the legal Western Emperor. Nevertheless, Romulus Augustus is often known as the last Western Roman Emperor. In 476, after being refused lands in Italy, Orestes' Germanic mercenaries under the leadership of the chieftain Odoacer captured and executed Orestes and took Ravenna, the Western Roman capital at the time, deposing Romulus Augustus. The whole of Italy was quickly conquered, and Odoacer was granted the title of patrician by Zeno, effectively recognizing his rule in the name of the Eastern Empire. Odoacer returned the Imperial insignia to Constantinople and ruled as King in Italy. Following Nepos' death Theodoric the Great, King of the Ostrogoths, conquered Italy with Zeno's approval.