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.
Justinian II
Justinian II (, "Ioustinianos II"; ; 668 – 11 December 711), surnamed the Rhinotmetos or Rhinotmetus (, "the slit-nosed"), was the last Byzantine Emperor of the , reigning from 685 to 695 and again from 705 to 711. Justinian II was an ambitious and passionate ruler who was keen to restore the Roman Empire to its former glories, but he responded poorly to any opposition to his will and lacked the finesse of his father, Constantine IV. Consequently, he generated enormous opposition to his reign, resulting in his deposition in 695 in a popular uprising, and he only returned to the throne in 705 with the help of a Bulgar and Slav army. His second reign was even more despotic than the first, and it too saw his eventual overthrow in 711, abandoned by his army who turned on him before killing him.