The Roman Empire began effectively with AUGUSTUS’ (the man who would later become Emperor) victory over Mark ANTONY and CLEOPATRA in 31 BC.
During the following centuries, Roman possessions outside Italy substantially expanded, and the complexity of the imperial bureaucracy resulted in a decline in the importance of Italy itself.
A growing number of emperors (whose allegiances lay elsewhere) were born outside Italy, and when Caracalla (AD 212 or 213) proclaimed an Edict which extended Roman citizenship to nearly all free provincials throughout the empire, Italy’s special status had all but disappeared.
The 7 emperors who reigned between 270 and 284 AD – also known as the “barracks emperors” – (Aurelian, Tacitus, Florianus, Probus, Carus, Carinus jointly with Numerianus and Carinus alone) were all chosen by the army.
Only Numerianus, who died during a march, and Carus who was killed in battle, died in an “ordinary” way. The other 5 emperors were killed by their own soldiers and generals. In an attempt to end the chaos of the “barracks emperors”, emperor Diocletian (284-305 AD) established an orderly succession process and divided the power and succession into two separate empires, the Eastern and the Western halves, the Eastern, being the senior emperor. As of 286 AD, Diocletian as the Eastern emperor was joined by Maximian (286-305) in the West. Both emperors abdicated in 305 AD.
Maximian was recalled in 306 AD by Galerius. In Subsequent years, that succession rule was bitterly disputed both in the East and the West. There were a total of 39 claimants to the imperial title between 305 AD and 474 AD, and only 5 emperors (Constantine I [312-337], Constantius II [350-361], Julian [361-363], Jovian [363-364] and Theodosius I [392-395]) ruled both the East and the West.