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The Roman Empire  was the post-Republican period of ancient Rome, consisting of large territorial holdings around the Mediterranean sea in Europe, North Africa and West Asia ruled by emperors. From the accession of Caesar Augustus to the military anarchy of the third century, it was a principate with Italy as metropole of the provinces and its city of Rome as sole capital (27 BC – 286 AD). The Roman Empire was then ruled by multiple emperors and divided into a Western Roman Empire, based in Milan and later Ravenna, and an Eastern Roman Empire, based in Nicomedia and later Constantinople. Rome remained the nominal capital of both parts until 476 AD, when it sent the imperial insignia to Constantinople (Byzantium in Ancient Greek) following the capture of Ravenna by the barbarians of Odoacer and the subsequent deposition of Romulus Augustus. The fall of the Western Roman Empire to Germanic kings, along with the hellenization of the Eastern Roman Empire into the Byzantine Empire, is conventionally used to mark the end of Ancient Rome and the beginning of the Middle Ages.

The previous Roman Republic, which had replaced Rome's monarchy in the 6th century BC, became severely destabilized in a series of civil wars and political conflict. In the mid-1st century BC Julius Caesar was appointed as perpetual dictator and then assassinated in 44 BC. Civil wars and proscriptions continued, culminating in the victory of Octavian, Caesar's adopted son, over Mark Antony and Cleopatra at the Battle of Actium in 31 BC. The following year Octavian conquered Ptolemaic Egypt, ending the Hellenistic period that had begun with the conquests of Alexander the Great of Macedon in the 4th century BC. Octavian's power was then unassailable and in 27 BC the Roman Senate formally granted him overarching power and the new title Augustus, effectively making him the first emperor.

The first two centuries of the Empire were a period of unprecedented stability and prosperity known as the Pax Romana ("Roman Peace"). It reached its greatest territorial expanse during the reign of Trajan (98–117 AD). A period of increasing trouble and decline began with the reign of Commodus. In the 3rd century, the Empire underwent a crisis that threatened its existence, but was reunified under Aurelian. In an effort to stabilize the Empire, Diocletian set up two different imperial courts in the Greek East and Latin West. Christians rose to power in the 4th century following the Edict of Milan in 313 and the Edict of Thessalonica in 380. Shortly after, the Migration Period involving large invasions by Germanic peoples and the Huns of Attila led to the decline of the Western Roman Empire. With the fall of Ravenna to the Germanic Herulians and the deposition of Romulus Augustulus in 476 AD by Odoacer, the Western Roman Empire finally collapsed and it was formally abolished by emperor Zeno in 480 AD. The Eastern Roman Empire, known in the post-Roman West as the Byzantine Empire, collapsed when Constantinople fell to the Ottoman Turks of Mehmed II in 1453.

Due to the Roman Empire's vast extent and long endurance, the institutions and culture of Rome had a profound and lasting influence on the development of language, religion, architecture, philosophy, law, and forms of government in the territory it governed, particularly Europe. The Latin language of the Romans evolved into the Romance languages of the medieval and modern world, while Medieval Greek became the language of the Eastern Roman Empire. Its adoption of Christianity led to the formation of Christendom during the Middle Ages. Greek and Roman art had a profound impact on the late medieval Italian Renaissance, while Rome's republican institutions influenced the political development of later republics such as the United States and France. The corpus of Roman law has its descendants in many legal systems of the world today, such as the Napoleonic Code. Rome's architectural tradition served as the basis for Neoclassical architecture.