The Roman Empire, (Old Latin: Imperium Rōmānum, Common Hellenic: Βασιλεία τῶν Ῥωμαίων) was the post-Republic period of the ancient Roman civilization. Its capital, the city of Rome was the largest city in the world c. 100 BC – c. 400 AD, with Konstantinople (New Rome) becoming the largest around 500 AD, 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.
Rise of the Empire:
The empire 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 AD, 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 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.
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 Konstantine I, who defeated his rivals and became the sole ruler of the empire. Constantine subsequently shifted the capital to Byzantium, which was renamed "Konstantinople" in his honour. 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 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 Roman 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, Augustulus was never recognized by his Eastern colleague, and separate rule in the Western part of the empire only ceased to exist upon the death of Julius Nepos, in 480. The Eastern Roman Empire endured for another millennium, 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 ancient history era, and one of the largest empires in world history. At its height under Trajan, it covered 5 million square kilometers. It held sway over an estimated 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 languages, cultures, religions, inventions, architectures, philosophies, laws 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. These efforts culminated in the foundation of the Holy Roman Empire, which is still here today.
Old Latin was the main language of the western half of the empire, and it had a strong presence throughout the entire realm. During the Roman period, Old Latin effected the extinction of almost all the other languages in its domain, and, by the end of the empire, enjoyed a hegemony in Western Europe which has never since been seen. When the empire fell, that large Old Latin-speaking area broke up into many separate states, all of whose dialects began to diverge. This divergence, the Christian colony in Independent Antarctica and the European colonization during and after the Renaissance, all resulted in the spread of the Romance Languages, now the largest and most widespread language group in the world.