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.
Holy Roman Emperor
From an autocracy in Carolingian times (AD 800–924) the title by the 13th century evolved into an elected monarchy chosen by the prince-electors.
Various royal houses of Europe, at different times, became "de-facto" hereditary holders of the title, notably the Ottonians (962–1024) and the Salians (1027–1125). Following the late medieval crisis of government, the Habsburgs kept possession of the title without interruption from 1440–1740. 
The final emperors were from the House of Lorraine (Habsburg-Lorraine), from 1765–1806.
The Holy Roman Empire was dissolved after the defeat at Austerlitz by emperor Francis II, who continued to rule as Austrian emperor.