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.
Majorian
A prominent general of the Late Roman army, Majorian deposed Emperor Avitus in 457 and succeeded him. Majorian was the last emperor to make a concerted effort to restore the Western Roman Empire with its own forces. Possessing little more than Italy, Dalmatia, and some territory in northern Gaul, Majorian campaigned rigorously for three years against the Empire's enemies. His successors until the fall of the Empire, in 476/480, were actually instruments of their barbarian generals, or emperors chosen and controlled by the Eastern Roman court.