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. 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
Roman Consuls above all) in conjunction with the senate.
 For various reasons, the 1st century BC 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 onwards, 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,
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 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 Antony and
Cleopatra at the
Battle of Actium in 31 BC. In 27 BC the
Senate and People of Rome made Octavian
princeps ("first citizen") with
imperium, thus beginning the
Principate (the first epoch of Roman imperial history, usually dated from 27 BC to AD 284), 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 sixty years of
Jewish–Roman wars in the second half of the 1st century and the first half of the 2nd century were exceptional in their duration and violence.
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—
Nero—before it yielded in 69 AD 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":
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
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
Aurelian (reigned 270–275) brought the empire back from the brink and stabilized it.
Diocletian completed the work of fully restoring the empire, but declined the role of princeps and became the first emperor to be addressed regularly as domine, "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
"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 a separate emperor, the
 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, 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
Valentinian dynasties, the empire was divided along an east–west axis, with dual power centres 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.
Western 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 Western Roman Empire in 476, when
Romulus Augustulus was
forced to abdicate to the
 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 empire in the East—often known as the
Byzantine Empire, but referred to in its time as the Roman Empire or by various other names—had a different fate. It survived for almost a millennium after the fall of its
Western counterpart and became the most stable Christian
realm during the Middle Ages. During the 6th century,
Justinian I reconquered
Northern Africa and Italy. But within a few years of Justinian's death, Byzantine possessions in Italy were greatly reduced by the
Lombards who settled in the peninsula.
 In the east, partially resulting from the destructive
Plague of Justinian, the Romans were threatened by the
rise of Islam, whose followers rapidly
conquered the territories of Syria,
Egypt during the
Byzantine-Arab Wars, and soon presented a direct
threat to Constantinople.
 In the following century, the Arabs also
captured southern Italy and Sicily.
 Slavic populations were also able to penetrate deep into the
The Roman (Byzantine) Empire c. 1263.
The Romans, however, managed to stop further Islamic expansion into their lands during the 8th century and, beginning in the 9th century, reclaimed parts of the conquered lands.
 In 1000 AD, the Eastern Empire was at its height:
Basil II reconquered Bulgaria and Armenia, culture and trade flourished.
 However, soon after, the expansion was abruptly stopped in 1071 with the Byzantine defeat in the
Battle of Manzikert. The aftermath of this important battle sent the empire into a protracted period of decline. Two decades of internal strife and
Turkic invasions ultimately paved the way for Emperor
Alexios I Komnenos to send a call for help to the Western European kingdoms in 1095.
The West responded with the
Crusades, eventually resulting in the
Sack of Constantinople by participants in the
Fourth Crusade. The conquest of Constantinople in 1204 fragmented what remained of the Empire into successor states, the ultimate victor being that of
 After the recapture of Constantinople by Imperial forces, the Empire was little more than a Greek state confined to the
Aegean coast. The Roman Empire finally collapsed when
Mehmed the Conqueror
conquered Constantinople on 29 May 1453.