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.
Roman Republic
Since the foundation of Rome, its rulers had been monarchs, elected for life by the patrician noblemen who made up the Roman Senate. The last Roman king was Lucius Tarquinius Superbus ("Tarquin the Proud"). In the traditional histories, Tarquin was expelled in 509 because his son, Sextus Tarquinius, had raped the noblewoman Lucretia, who afterwards took her own life. Lucretia's father; her husband, Lucius Tarquinius Collatinus; and Tarquin's own nephew, Lucius Junius Brutus, mustered support from the Senate and army, and forced Tarquin into exile in Etruria.