The Roman Empire 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 extended and between c. 100 BC- 400 AD was the largest city in the world, whereas Constantinople (New Rome) retained the first position around 500 AD while the Empire's population grew into an estimated 50 to 90 million inhabitants (roughly 20% of the world's population). 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 now 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.
The imperial successor to the Republic lasted approximately 1400 years. 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 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 68, the Empire suffered a period 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 69, establishing the Flavian dynasty, before being succeeded by his son Titus, who opened the Colosseum shortly after the eruption of Mt. 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.
A period of increasing trouble and decline began with the reign of Commodus. Commodus' assassination in 192 triggered the Year of the Five Emperors, of which Septimius Severus emerged victorious. The assassination of Alexander Severus in 235 led to the Crisis of the Third Century in which 26 men were declared Emperor by the Roman Senate over a fifty-year period. It was not until the reign of Diocletian that the Empire was fully stabilized with the introduction of the Tetrarchy, which saw four Emperors rule the Empire at once. This arrangement was ultimately unsuccessful, leading to a civil war that was finally ended by Constantine I, who defeated his rivals and became the sole ruler of the Empire. Constantine subsequently shifted the capital to Byzantium, which was renamed Constantinople in his own honor. It remained the capital of the east until its demise. Constantine also adopted Christianity which later became the official state religion of the Empire. This eastern part of the empire (known later as the Byzantine Empire) remained one of the leading powers in the world alongside its arch-rival the Sassanid Persian Empire, which had inherited a centuries-old Roman-Persian conflict from its predecessor the Parthians. Following the death of Theodosius I, the last Emperor to rule a united Empire, the dominion of the Empire was gradually eroded by abuses of power, civil wars, barbarian migrations and invasions, military reforms and economic depression. The Sack of Rome in 410 by the Visigoths and again in 455 by the Vandals accelerated the Western Empire's decay, while the deposition of the Emperor Romulus Augustulus in 476 by Odoacer is generally accepted to mark the end of the Empire in the west. However, with Romulus Augustulus technically being a usurper, the Western part of the empire only truly legally ceased to exist upon the death of the true Emperor Julius Nepos in 480. The Eastern Roman Empire endured for another thousand years, eventually falling to the Ottoman Turks in 1453.
The Roman Empire was among the most powerful economic, cultural, political and military forces in the world of its time. It was the largest empire of the classical antiquity period, and one of the largest empires in world history. At its height under Trajan, it covered 5 million square kilometers and held sway over some 70 million people, at that time 21% of the world's entire population. The longevity and vast extent of the Empire ensured the lasting influence of Latin and Greek language, culture, religion, inventions, architecture, philosophy, law and forms of government on the Empire's descendants. Throughout the European medieval period, attempts were even made to establish successors to the Roman Empire, including the Crusader state, the Empire of Romania and the Holy Roman Empire. By means of European expansionism through the Spanish, French, Portuguese, Dutch, Italian, Russian, German, British and Belgian Empires, Greco-Roman culture was spread on a worldwide scale, playing a significant role in the development of the modern world.