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.
Sack of Rome (410)
The sack was a culmination of many terminal problems facing the Western Roman Empire. Domestic rebellions and usurpations weakened the Empire in the face of external invasions. These factors would permanently harm the stability of the Roman Empire in the west. The Roman army meanwhile became increasingly barbarian and disloyal to the Empire. A more severe sack of Rome by the Vandals followed in 455, and the Western Roman Empire finally collapsed in 476 when the Germanic Odovacer removed the last Western Roman Emperor, Romulus Augustulus, and declared himself King of Italy.