History of the Roman Empire

The history of the Roman Empire covers the history of ancient Rome from the traditional end of the Roman Republic in 27 BC until the abdication of Romulus Augustulus in AD 476 in the West, and the Fall of Constantinople in the East in 1453. Ancient Rome became a territorial empire while still a republic, but was then ruled by emperors beginning with Octavian Augustus, the final victor of the republican civil wars.

Rome had begun expanding shortly after the founding of the Republic in the 6th century BC, though it did not expand outside the Italian Peninsula until the 3rd century BC, during the Punic Wars, after which the Republic expanded across the Mediterranean. Civil war engulfed Rome in the mid-1st century BC, first between Julius Caesar and Pompey, and finally between Octavian (Caesar's grand-nephew) and Mark Antony. Antony was defeated at the Battle of Actium in 31 BC, leading to the annexation of Egypt. In 27 BC, the Senate gave Octavian the titles of Augustus ("venerated") and Princeps ("foremost"), thus beginning the Principate, the first epoch of Roman imperial history. Augustus' name was inherited by his successors, as well as his title of Imperator ("commander"), from which the term "emperor" is derived. Early emperors avoided any association with the ancient kings of Rome, instead presenting themselves as leaders of the Republic.

The success of Augustus in establishing principles of dynastic succession was limited by his outliving a number of talented potential heirs; the Julio-Claudian dynasty lasted for four more emperors—Tiberius, Caligula, Claudius, and Nero—before it yielded in AD 69 to the strife-torn Year of the Four Emperors, from which Vespasian emerged as victor. Vespasian became the founder of the brief Flavian dynasty, to be followed by the Nerva–Antonine dynasty which produced the "Five Good Emperors": Nerva, Trajan, Hadrian, Antoninus Pius and the philosophically inclined Marcus Aurelius. In the view of the Greek historian Cassius Dio, a contemporary observer, the accession of the emperor Commodus in AD 180 marked the descent "from a kingdom of gold to one of rust and iron"—a famous comment which has led some historians, notably Edward Gibbon, to take Commodus' reign as the beginning of the decline of the Roman Empire.

In 212, during the reign of Caracalla, Roman citizenship was granted to all freeborn inhabitants of the Empire. Despite this gesture of universality, the Severan dynasty was tumultuous—an emperor's reign was ended routinely by his murder or execution—and following its collapse, the Empire was engulfed by the Crisis of the Third Century, a 50-year period of invasions, civil strife, economic disorder, and epidemic disease. In defining historical epochs, this crisis is typically viewed as marking the start of the Later Roman Empire, and also the transition from Classical to Late antiquity. In the reign of Philip the Arab (r. 244–249), Rome celebrated its thousandth anniversary with the Saecular Games. Diocletian (r. 284–305) restored stability to the empire, modifying the role of princeps and adopting the style of dominus, "master" or "lord", thus beginning the period known as the Dominate. Diocletian's reign also brought the Empire's most concerted effort against Christianity, the "Great Persecution". The state of absolute monarchy that began with Diocletian endured until the fall of the Eastern Roman Empire in 1453.

In 286, the empire was split into two halves, each with its own emperor and court. The empire was further divided into four regions in 293, beginning the Tetrarchy. By this time, Rome itself was reduced to a symbolic status, as emperors ruled from different cities. Diocletian abdicated voluntarily along with his co-augustus, but the Tetrarchy almost immediately fell apart. The civil wars ended in 324 with the victory of Constantine I, who became the first emperor to convert to Christianity and who founded Constantinople as a new capital for the whole empire. The reign of Julian, who attempted to restore Classical Roman and Hellenistic religion, only briefly interrupted the succession of Christian emperors of the Constantinian dynasty. During the decades of the Valentinianic and Theodosian dynasties, the established practice of dividing the empire in two was continued. Theodosius I, the last emperor to rule over both the Eastern empire and the whole Western empire, died in 395 after making Christianity the official religion of the Empire.

The Western Roman Empire began to disintegrate in the early 5th century as the Germanic migrations and invasions of the Migration Period overwhelmed the capacity of the Empire to assimilate the immigrants and fight off the invaders. Most chronologies place the end of the Western Roman Empire in 476, when Romulus Augustulus was forced to abdicate to the Germanic warlord Odoacer. The Eastern empire exercised diminishing control over the west over the course of the next century and was reduced to Anatolia and the Balkans by the 7th. The empire in the east—known today as the Byzantine Empire, but referred to in its time as "Roman"—ended in 1453 with the death of Constantine XI and the fall of Constantinople to the Ottoman Turks (see History of the Byzantine Empire).