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.
Coronations in Europe
Since Charlemagne in 800, Holy Roman Emperors were crowned by the pope until 1530, when Charles V became the last Holy Roman Emperor to be crowned by the Pope, at Bologna. Thereafter, until the abolition of the empire in 1806, Imperial coronations were held in Frankfurt and were performed by the Spiritual Princes-Electors,the Archbishops of Cologne, Mainz and Trier. Later rulers simply proclaimed themselves "Electus Romanorum Imperator" or "Elected Emperor of the Romans", without the formality of a coronation by the Pontiff.