History of the Roman Empire

The history of the Roman Empire covers the history of ancient Rome from the fall 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 AD 1453. Ancient Rome became a territorial empire while still a republic, but was then ruled by Roman emperors beginning with Augustus (r. 27 BC – AD 14), becoming the Roman Empire following the death of the last republican dictator, the first emperor's adoptive father Julius Caesar.

Rome had begun expanding shortly after the founding of the Roman Republic in the 6th century BC, though it did not expand outside the Italian Peninsula until the 3rd century BC. Civil war engulfed the Roman state in the mid-1st century BC, first between Julius Caesar and Pompey, and finally between Octavian and Mark Antony. Antony was defeated at the Battle of Actium in 31 BC. In 27 BC, the Senate made Octavian imperator ("commander") thus beginning the Principate, the first epoch of Roman imperial history usually dated from 27 BC to AD 284; later that year they awarded him the name Augustus, "the venerated". Subsequent emperors all took this name as the imperial title Augustus.

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 Dio Cassius, 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 Roman Empire was engulfed by the Crisis of the Third Century, a 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 antiquity to Late antiquity. In the reign of Philip the Arab (r. 244–249), Rome celebrated the thousandth anniversary of her founding by Romulus and Remus with the Saecular Games. Diocletian (r. 284–305) restored stability to the empire, modifying the role of princeps and becoming the first emperor to be addressed by Roman citizens as domine, "master" or "lord" or referred to as dominus noster "our lord". Diocletian's reign also brought the Empire's most concerted effort against the perceived threat of Christianity, the "Great Persecution". The state of absolute monarchy that began with Diocletian endured until the fall of the Eastern Roman Empire in 1453.

Diocletian divided the empire into four regions, each ruled by an emperor (the Tetrarchy). Confident that he fixed the disorders plaguing Rome, he abdicated along with his co-augustus, and the Tetrarchy eventually collapsed in the civil wars of the Tetrarchy. Order was eventually restored by the victories of Constantine, who became the first emperor to convert to Christianity, and who founded Constantinople as a new capital for the empire after defeating his co-emperor Licinius. The reign of Julian, who under the influence of his adviser Mardonius 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 multiple emperors was continued. Theodosius I, the last emperor to rule over both the Eastern empire and the whole Western empire, died in AD 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. By placing himself under the rule of the eastern emperor Zeno, rather than naming himself or a puppet ruler as emperor, as other Germanic chiefs, had done, Odoacer ended the separate Roman government of the Western empire. The Eastern empire exercised diminishing control over the west over the course of the next century. The empire in the east—known today as the Byzantine Empire, but referred to in its time as the "Roman Empire" or by various other names—ended in 1453 with the death of Constantine XI and the fall of Constantinople to the Ottoman Turks.