The Roman Empire was the post-Republican phase of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean. The term is used to describe the Roman state during and after the time of the first emperor, Augustus. The nearly 500-year-old Roman Republic, which preceded it, had been weakened by several civil wars.[nb 2] Several events are commonly proposed to mark the transition from Republic to Empire, including Julius Caesar's appointment as perpetual dictator (44 BC), the victory of Octavian at the Battle of Actium (2 September 31 BC), and the Roman Senate's granting to Octavian the honorific Augustus (4 January 27 BC).[nb 3]
The Latin term Imperium Romanum (Roman Empire), probably the best-known Latin expression where the word imperium denotes the sphere of human life (for example some countries - lands with people) subdued to military commander - imperator, under Roman rule. Roman expansion began in the days of the Republic, but reached its zenith under Emperor Trajan. At this territorial peak, the Roman Empire controlled approximately 6,500,000 km² of land surface. Because of the Empire's vast extent and long endurance, Roman influence upon the language, religion, architecture, philosophy, law, and government of nations around the world lasts to this day.
In the late 3rd century AD, Diocletian established the practice of dividing authority between four co-emperors, in order to better administer the vast territory. In 330 AD, the first Christian Emperor Constantine I, founded Constantinople as new capital of the Empire. In 392 AD, Emperor Theodosius appointed as co-emperors, his elder son Arcadius for the east part of the Empire and his younger son Honorius for the west part of the Empire. After the death of Theodosius in 395 AD, the empire was finally divided to its east part having Constantinople as capital, and to its west part having Mediolanum (since 402) then Ravenna as capital.
The Western Roman Empire collapsed in 476 as Romulus Augustus was forced to abdicate by Odoacer. The Eastern Roman or Byzantine Empire endured until 1453 with the death of Constantine XI and the capture of Constantinople by the Ottoman Turks led by Mehmed II.