<Tr> <Td> Western Roman Empire </Td> <Td> </Td> </Tr> <Tr> <Td> Eastern Roman Empire </Td> <Td> </Td> </Tr> <P> The Roman Empire (Latin: Imperium Rōmānum, Classical Latin: (ɪmˈpɛ. ri. ũː roːˈmaː. nũː); Koine and Medieval Greek: Βασιλεία τῶν Ῥωμαίων, tr . Basileia tōn Rhōmaiōn) was the post-Roman Republic period of the ancient Roman civilization, characterized by government headed by emperors and large territorial holdings around the Mediterranean Sea in Europe, Africa and Asia . The city of Rome was the largest city in the world c. 100 BC--c . AD 400, with Constantinople (New Rome) becoming the largest around AD 500, and the Empire's populace grew to an estimated 50 to 90 million inhabitants (roughly 20% of the world's population at the time). The 500 - year - old republic which preceded it was severely destabilized in a series of civil wars and political conflict, during which Julius Caesar was appointed as perpetual dictator and then assassinated in 44 BC . Civil wars and executions continued, culminating in the victory of Octavian, Caesar's adopted son, over Mark Antony and Cleopatra at the Battle of Actium in 31 BC and the annexation of Egypt . Octavian's power was then unassailable and in 27 BC the Roman Senate formally granted him overarching power and the new title Augustus, effectively marking the end of the Roman Republic . </P> <P> The imperial period of Rome lasted approximately 1,500 years compared to the 500 years of the Republican era . The first two centuries of the empire's existence were a period of unprecedented political stability and prosperity known as the Pax Romana, or "Roman Peace". Following Octavian's victory, the size of the empire was dramatically increased . After the assassination of Caligula in AD 41, the Senate briefly considered restoring the republic, but the Praetorian Guard proclaimed Claudius emperor instead . Under Claudius, the empire invaded Britannia, its first major expansion since Augustus . After Claudius' successor, Nero, committed suicide in AD 68, the empire suffered a series of brief civil wars, as well as a concurrent major rebellion in Judea, during which four different legionary generals were proclaimed emperor . Vespasian emerged triumphant in AD 69, establishing the Flavian dynasty, before being succeeded by his son Titus, who opened the Colosseum shortly after the eruption of Mount Vesuvius . His short reign was followed by the long reign of his brother Domitian, who was eventually assassinated . The Senate then appointed the first of the Five Good Emperors . The empire reached its greatest extent under Trajan, the second in this line . </P>

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