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.
Probus (emperor)
According to the favorable treatment of Gibbon (whose account is largely derived from the "Augustan History"), Probus was the last of the benevolent constitutional emperors of Rome. While his successor Carus (Imp. 281-284) simply disdained to seek the senate's confirmation of his title, the latter's successor Diocletian (Imp. 284-305) took active measures to undermine its authority, and established the autocratic nature and divine deriviation of the Imperial power. Never again, after Diocletian's reforms, would the Roman senate play an active role in the management of the empire. On the military sphere, Probus' victories continued the succession of martial Illyrian emperors begun by Claudius Gothicus, which restored the military supremacy of Rome after defeats sustained during the crisis of the third century.Attribution: