The Roman Empire was the post-Republican period of ancient Rome.
As a polity, it included large territorial holdings around the Mediterranean Sea in Europe,
North Africa, and Western Asia, and was ruled by emperors.
From the accession of Caesar Augustus
as the first Roman emperor to the military anarchy of the 3rd century,
it was a Principate with Italia as the metropole of its provinces and the city of Rome
as its sole capital.
The Empire was later ruled by multiple emperors
who shared control over the Western Roman Empire and the Eastern Roman Empire.
The imperial seat moved from Rome to Byzantium and following the collapse of the West in AD 476,
it became its sole capital as Constantinople.
The adoption of Christianity as the state church of the Roman Empire in AD 380
and the fall of the Western Roman Empire to Germanic kings conventionally marks
the end of classical antiquity and the beginning of the Middle Ages.
Because of these events, along with the prevalence of Greek instead of Latin,
some historians distinguish the medieval Roman Empire
that remained in the Eastern provinces as the Byzantine Empire.
The predecessor state of the Roman Empire, the Roman Republic,
became severely destabilized in civil wars and political conflicts.
In the middle of the 1st century BC, Julius Caesar was appointed as dictator perpetuo
("dictator in perpetuity"),
and then assassinated in 44 BC.
Civil wars and proscriptions continued,
eventually culminating in the victory
of Octavian over Mark Antony and Cleopatra at the Battle of Actium in 31 BC.
The following year, Octavian conquered the Ptolemaic Kingdom in Egypt,
ending the Hellenistic period that had begun with
the 4th century BC conquests of Alexander the Great.
Octavian's power became unassailable
and the Roman Senate granted him overarching power (imperium)
and the new title of Augustus, making him the first Roman emperor.
The vast Roman territories were organized in senatorial and imperial provinces except Italy,
which continued to serve as a metropole.
The first two centuries of the Roman Empire
saw a period of unprecedented stability
and prosperity known as the Pax Romana (lit. 'Roman Peace').
Rome reached its greatest territorial expanse during the reign of Trajan (AD 98-117);
a period of increasing trouble
and decline began with the reign of Commodus (177-192).
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, often from the legions, led the Empire.
It was reunified under Aurelian (r. 270-275).
To stabilize it,
Diocletian set up two different imperial courts in the Greek East and Latin West in 286;
Christians rose to positions of power in the 4th century following the Edict of Milan of 313.
Shortly after, 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 AD 476 by Odoacer,
the Western Roman Empire finally collapsed;
the Eastern Roman emperor Zeno formally abolished it in AD 480.
The Eastern Roman Empire survived for another millennium,
until Constantinople fell in 1453 to the Ottoman Turks under Mehmed II.
Due to the Roman Empire's vast extent and long endurance,
the institutions and culture of Rome
had a profound and lasting influence on the development of language,
religion, art, architecture, literature, philosophy, law, and forms
of government in the territory it governed.
The Latin language of the Romans evolved into the Romance languages
of the medieval and modern world, while Medieval Greek
became the language of the Eastern Roman Empire.
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 also had a strong influence on Islamic architecture.
The rediscovery of Greek and Roman science and technology
which also formed the basis for Islamic science)
in Medieval Europe led to the Scientific Renaissance and Scientific Revolution.
The corpus of Roman law has its descendants in many modern legal systems of the world,
such as the Napoleonic Code of France,
while Rome's republican institutions have left an enduring legacy,
influencing the Italian city-state republics of the medieval period,
as well as the early United States and other modern democratic republics.