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 expansion in Italy
The last king of Rome, Tarquinius Superbus, is portrayed by the sources as a powerful, but tyrannical, king. He is said to have taken Pometia by storm and Gabii by ruse, colonized Signia and Circeii, and to have organized the Latin states into an alliance under his own leadership. However Tarquinius was deposed by a conspiracy led by two of his relatives, Lucius Junius Brutus and Lucius Tarquinius Collatinus. These two then founded the Roman Republic with themselves as its first two consuls; traditionally the overthrow of the monarchy has been dated to 509 BC. Tarquinius is supposed to have sought refuge with king Lars Porsenna of Clusium, who marched against Rome to reinstate the king. After various heroic acts by brave defenders of Rome, Porsenna gave up and instead attacked Aricia, but was defeated by an alliance of Latin cities and the Greek colony of Cumae. Modern historians have had difficulties accepting this tale and have instead proposed that Porsenna actually captured Rome and deposed Tarquinius and replaced him with two leading aristocrats. With leadership of the city divided, Rome would have been easier for Porsenna to control.