The Roman Empire was the post-Republican state of ancient Rome. It included territory around the Mediterranean in Europe, North Africa, and Western Asia, and was ruled by emperors. The adoption of Christianity as the state church in 380 and the fall of the Western Roman Empire conventionally marks the end of classical antiquity and the beginning of the Middle Ages.

Rome had expanded its rule to most of the Mediterranean and beyond, but became severely destabilized in civil wars and political conflicts which culminated in the victory of Octavian over Mark Antony and Cleopatra at the Battle of Actium in 31 BC and the subsequent conquest of the Ptolemaic Kingdom in Egypt.

The Roman Senate granted Octavian overarching power (imperium) and the new title of Augustus, marking his accession as the first Roman emperor of a monarchy with Rome as its sole capital. The vast Roman territories were organized in senatorial and imperial provinces.

The first two centuries of the Empire saw a period of unprecedented stability and prosperity known as the Pax Romana (lit. ’Roman Peace’). Rome reached its greatest territorial expanse under Trajan (AD 98–117); a period of increasing trouble and decline began under Commodus (180–192).

In the 3rd century, the Empire underwent a crisis that threatened its existence, as the Gallic and Palmyrene Empires broke away from the Roman state, and a series of short-lived emperors led the Empire. It was reunified under Aurelian (r. 270–275).

Diocletian set up two different imperial courts in the Greek East and Latin West in 286; Christians rose to power in the 4th century following the Edict of Milan. The imperial seat moved from Rome to Byzantium in 330, renamed Constantinople after Constantine the Great.

The Migration Period, involving large invasions by Germanic peoples and by the Huns of Attila, led to the decline of the Western Roman Empire.

With the fall of Ravenna to the Germanic Herulians and the deposition of Romulus Augustus in 476 by Odoacer, the Western Roman Empire finally collapsed. The Eastern Roman Empire survived for another millennium with Constantinople as its sole capital, until the city’s fall in 1453.

Due to the Empire’s extent and endurance, its institutions and culture had a lasting influence on the development of language, religion, art, architecture, literature, philosophy, law, and forms of government in its territories. Latin evolved into the Romance languages, while Medieval Greek became the language of the East. The Empire’s adoption of Christianity led to the formation of medieval Christendom.

Roman and Greek art had a profound impact on the Italian Renaissance. Rome’s architectural tradition served as the basis for Romanesque, Renaissance and Neoclassical architecture, and influenced Islamic architecture.

The rediscovery of classical science and technology (which formed the basis for Islamic science) in medieval Europe led to the Scientific Renaissance and Scientific Revolution.

Many modern legal systems, such as the Napoleonic Code, descend from Roman law, while Rome’s republican institutions have influenced the Italian city-state republics of the medieval period, the early United States, and modern democratic republics.

History

Rome had begun expanding shortly after the founding of the Roman Republic in the 6th century BC, though not outside the Italian peninsula until the 3rd century BC.

Thus, it was an “empire” (a great power) long before it had an emperor. The Republic was not a nation-state in the modern sense, but a network of self-ruled towns (with varying degrees of independence from the Senate) and provinces administered by military commanders.

It was governed by annually elected magistrates (Roman consuls above all) in conjunction with the Senate. 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, meaning “command” (though typically in a military sense).

Occasionally, successful consuls were given the honorary title imperator (commander); this is the origin of the word emperor, since this title was always bestowed to the early emperors.

Rome suffered a long series of internal conflicts, conspiracies, and civil wars from the late second century BC—Crisis of the Roman Republic—while greatly extending its power beyond Italy. 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 Mark Antony and Caesar’s adopted son Octavian. Antony and Octavian’s division of the Roman world did not last and Octavian’s forces defeated those of Mark Antony and Cleopatra at the Battle of Actium in 31 BC.

In 27 BC the Senate 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 title Augustus (“the venerated”).

Although the republic stood in name, Augustus had all meaningful authority.[27] Since his rule 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.

Pax Romana

The so-called “Five Good Emperors” of 96–180 AD

Nerva (r. 96–98)

Trajan (r. 98–117)

Hadrian (r. 117–138)

Antoninus Pius (r. 138–161)

Marcus Aurelius (r. 161–180)

The 200 years that began with Augustus’s rule is traditionally regarded as the Pax Romana (“Roman Peace”). 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 and put down “mercilessly and swiftly”.

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-torn Year of the Four Emperors, from which Vespasian emerged as victor.

Vespasian became the founder of the brief Flavian dynasty, followed by the Nerva–Antonine dynasty which produced the “Five Good Emperors”: Nerva, Trajan, Hadrian, Antoninus Pius, and Marcus Aurelius.

Fall in the West and survival in the East

The Barbarian Invasions consisted of the movement of (mainly) ancient Germanic peoples into Roman territory. Historically, this event marked the transition between classical antiquity and the Middle Ages.

In the view of contemporary Greek historian Cassius Dio, the accession of Commodus in 180 marked the descent “from a kingdom of gold to one of rust and iron”—a comment which has led some historians, notably Edward Gibbon, to take Commodus’ reign as the beginning of the Empire’s decline.

In 212, during the reign of Caracalla, Roman citizenship was granted to all freeborn inhabitants of the empire. 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 period of invasions, civil strife, economic disorder, and plague.

In defining historical epochs, this crisis sometimes marks the transition from Classical to Late Antiquity. Aurelian (r. 270–275) stabilized the empire and Diocletian completed the work of fully restoring it in 285, but rejected the role of princeps and assumed the title of dominus (“lord”), thus starting the period known as the Dominate.

Diocletian’s reign brought the empire’s most concerted effort against the perceived threat of Christianity, the “Great Persecution”.

Diocletian divided the empire into four regions, each ruled by a separate tetrarch. Confident that he fixed the disorder plaguing Rome, he abdicated along with his co-emperor, but the Tetrarchy collapsed shortly after. 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 Constantinian and Valentinian dynasties, the empire was divided along an east–west axis, with dual power centres in Constantinople and Rome.

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. Theodosius I, the last emperor to rule over both East and West, died in 395 after making Christianity the state religion.

The Roman Empire by 476, noting western and eastern divisions

The Western Roman Empire began to disintegrate in the early 5th century. The Romans were successful in fighting off all invaders, most famously Attila, but 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.

Odoacer ended the Western Empire by declaring Zeno sole emperor and placing himself as Zeno’s nominal subordinate. In reality, Italy was ruled by Odoacer alone.

The Eastern Roman Empire, called the Byzantine Empire by later historians, continued until the reign of Constantine XI Palaiologos.

The last Roman emperor died in battle in 1453 against Mehmed II and his Ottoman forces during the siege of Constantinople. Mehmed II adopted the title of caesar in an attempt to claim a connection to the Empire.