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 the last Emperor in 476 AD. Rome had begun expanding shortly after the founding of the Republic in the 6th century BC, though didn’t expand outside of Italy 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 and People of Rome made Octavian imperator (“commander”) thus beginning the Principate (the first epoch of Roman imperial history, usually dated from 27 BC to 284 AD), and gave him the name Augustus (“the venerated”). 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 69 AD to the strife-tornYear of 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 180 AD 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. But 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 plague. In defining historical epochs, this crisis is sometimes viewed as marking the transition from Classical Antiquity to Late Antiquity. Diocletian (reigned 284–305) brought the Empire back from the brink, but declined the role of princeps and became the first emperor to be addressed regularly asdomine, “master” or “lord”. This marked the end of the Principate, and the beginning of the Dominate. 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 Western Roman Empire in 476.
Diocletian divided the empire into four regions, each ruled by a separate Emperor (the Tetrarchy). Confident that he fixed the disorders that were plaguing Rome, he abdicated along with his co-emperor, and the Tetrarchy soon collapsed. Order was eventually restored by Constantine, who became the first emperor to convert to Christianity, and who established Constantinople as the new capital of the eastern empire. During the decades of the Constantinian and Valentinian dynasties, the Empire was divided along an east–west axis, with dual power centers in Constantinople and Rome. The reign of Julian, who attempted to restore Classical Roman and Hellenistic religion, only briefly interrupted the succession of Christian emperors. Theodosius I, the last emperor to rule over both East and West, died in 395 AD after making Christianity the official religion of the Empire.
The Roman Empire began to disintegrate in the early 5th century as Germanic migrations and invasions overwhelmed the capacity of the Empire to assimilate the migrants and fight off the invaders. The Romans were successful in fighting off all invaders, most famously Attila the Hun, though the Empire had assimilated so many Germanic peoples of dubious loyalty to Rome that the Empire started to dismember itself. 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, rather than naming himself Emperor (as other Germanic chiefs had done after deposing past Emperors), Odoacer ended the Western Empire by ending the line of Western Emperors. 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.