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.
Fall of the Western Roman Empire
By convention, the Western Roman Empire is deemed to have ended on 4 September 476, when Odoacer deposed Romulus Augustulus and proclaimed himself ruler of Italy, but this convention is subject to many qualifications. In Roman constitutional theory, the Empire was still simply united under one emperor, implying no abandonment of territorial claims. In areas where the convulsions of the dying Empire had made organized self-defence legitimate, rump states continued under some form of Roman rule after 476. Julius Nepos still claimed to be Emperor of the West and controlled Dalmatia until his murder in 480. Syagrius son of Aegidius ruled the Domain of Soissons until his murder in 487. The indigenous inhabitants of Mauretania developed kingdoms of their own, independent of the Vandals, with strong Roman traits. They again sought Imperial recognition with the reconquests of Justinian I, and they put up effective resistance to the Muslim conquest of the Maghreb. While the civitates of Britannia sank into a level of material development inferior even to their pre-Roman Iron Age ancestors, they maintained identifiably Roman traits for some time, and they continued to look to their own defence as Honorius had authorized.