Capitals: Rome, Mediolanum, Ravenna
Official Languages: Latin, Greek
Established: 27 BC/BCE
Disestablished: 476 AD/CE
Rome had begun expanding shortly after the founding of the republic in the 6th century BC/BCE, though it did not expand outside the Italian peninsula until the 3rd century BC/BCE. Then, it was an "empire" long before it had an emperor. The Roman Republic was not a nation-state in the modern sense, but a network of towns left to rule themselves (though with varying degrees of independence from the Roman Senate) and provinces administered by military commanders. It was ruled, not by emperors, but by annually elected magistrates (Roman Consuls above all) in conjunction with the Senate. For various reasons, the 1st century BC/BCE was a time of political and military upheaval, which ultimately led to rule by emperors. The consuls' military power rested in the Roman legal concept of imperium, which literally means "command" (though typically in a military sense). Occasionally, successful consuls were given the honorary title imperator (commander), and this is the origin of the word emperor (and empire) since this title (among others) was always bestowed to the early emperors upon their accession.
Rome suffered a long series of internal conflicts, conspiracies and civil wars from the late second century BC/BCE onward, while greatly extending its power beyond Italy. This was the period of the Crisis of the Roman Republic. Towards the end of this era, in 44 BC/BCE, Julius Caesar was briefly perpetual dictator before being assassinated. The faction of his assassins was driven from Rome and defeated at the Battle of Philippi in 42 BC/BCE by an army led by Mark Antony and Caesar's adopted son Octavian. Antony and Octavian's division of the Roman world between themselves did not last and Octavian's forces defeated those of Mark Antony and Cleopatra at the Battle of Actium in 31 BC/BCE, ending the Final War of the Roman Republic. In 27 BC/BCE the Senate and People of Rome made Octavian princeps ("first citizen") with proconsular imperium, 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"). Though the old constitutional machinery remained in place, Augustus came to predominate it. Although the republic stood in name, contemporaries of Augustus knew it was just a veil and that Augustus had all meaningful authority in Rome. Since his rule ended a century of civil wars and began an unprecedented period of peace and prosperity, he was so loved that he came to hold the power of a monarch de facto if not de jure. During the years of his rule, a new constitutional order emerged (in part organically and in part by design), so that, upon his death, this new constitutional order operated as before when Tiberius was accepted as the new emperor.
The 200 years that began with Augustus's rule is traditionally regarded as the Pax Romana ("Roman Peace"). During this period, the cohesion of the empire was furthered by a degree of social stability and economic prosperity that Rome had never before experienced. Uprisings in the provinces were infrequent, but put down "mercilessly and swiftly" when they occurred. 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/CE to the strife-torn Year 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/CE 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 AD/CE, 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. Aurelian (reigned 270–275) brought the empire back from the brink and stabilized it.
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 the Great, and who established Constantinople as the new capital of the Byzantine Empire. During the decades of the Constantinian and Valentinian dynasties, the empire was divided along an east–west axis, with dual power centres in Constantinople and Rome. The reign of Julian, who under the influence of his adviser Mardonius attempted to restore Classical Roman and Hellenistic religions.
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, 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 Roman Empire in 476, when Romulus Augustus was forced to abdicate to the Germanic warlord Odoacer. By placing himself under the rule of the Byzantine Emperor, rather than naming himself emperor (as other Germanic chiefs had done after deposing past emperors), Odoacer ended the Roman Empire, by sending the imperial regalia to the Byzantine Emperor Zeno in effect declaring him sole emperor, and placing Odoacer himself as his nominal subordinate, though in reality Italy was now ruled by Odoacer alone. The Byzantine Empire continued to exist until the reign of Constantine XI Palaiologos who became the last Roman Emperor on 29 May 1453 after dying in battle during the Siege of Constantinople against Mehmed II or "the Conqueror" and his Ottoman forces, ending the Byzantine Empire, though Mehmed II would himself also claim the title of caesar or Kayser-i Rum in an attempt to claim a connection to the Roman Empire.