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The Roman Empire ruled the Mediterranean and much of Europe, Western Asia and North Africa. The Romans conquered most of this during the Republic, and it was ruled by emperors following Octavian's assumption of effective sole rule in 27 BC. The western empire collapsed in 476 AD, but the eastern empire lasted until the fall of Constantinople in 1453.

By 100 BC, the city of Rome had expanded its rule from the Italian peninsula to most of the Mediterranean and beyond. However, it was severely destabilised by 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. In 27 BC, the Roman Senate granted Octavian overarching military power (imperium) and the new title of Augustus, marking his accession as the first Roman emperor. The vast Roman territories were organized into senatorial provinces, governed by proconsuls who were appointed by lot annually, and imperial provinces, which belonged to the emperor but were governed by legates.

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 extent under Trajan (r. 98–117 AD), but a period of increasing trouble and decline began under Commodus (r. 180–192). In the 3rd century, the Empire underwent a 49-year crisis that threatened its existence due to civil war, plagues and barbarian invasions. The Gallic and Palmyrene empires broke away from the state and a series of short-lived emperors led the Empire, which was later reunified under Aurelian (r. 270–275). The civil wars ended with the victory of Diocletian (r. 284–305), who set up two different imperial courts in the Greek East and Latin West. Constantine the Great (r. 306–337), the first Christian emperor, moved the imperial seat from Rome to Byzantium in 330, and renamed it Constantinople. 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 Empire finally collapsed. The Byzantine (Eastern Roman) Empire survived for another millennium with Constantinople as its sole capital, until the city's fall in 1453.[f]

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 across its territories. Latin evolved into the Romance languages while Medieval Greek became the language of the East. The Empire's adoption of Christianity resulted in 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, influencing Islamic architecture. The rediscovery of classical science and technology (which formed the basis for Islamic science) in medieval Europe contributed to the Scientific Renaissance and Scientific Revolution. Many modern legal systems, such as the Napoleonic Code, descend from Roman law. Rome's republican institutions have influenced the Italian city-state republics of the medieval period, the early United States, and modern democratic republics.

The Byzantine Empire, also known as the Eastern Roman Empire, was the continuation of the Roman Empire centred on Constantinople during late antiquity and the Middle Ages. Having survived the events that caused the fall of the Western Roman Empire in the 5th century AD, it endured until the fall of Constantinople to the Ottoman Empire in 1453. The term 'Byzantine Empire' was coined only after its demise; its citizens used the term 'Roman Empire' and called themselves 'Romans'.

During the early centuries of the Roman Empire, the western provinces were Latinised, but the eastern parts kept their Hellenistic culture. Constantine I (r. 324–337) legalised Christianity and moved the capital to Constantinople. Theodosius I (r. 379–395) made Christianity the state religion and Greek gradually replaced Latin for official use. The empire adopted a defensive strategy and, throughout its remaining history, experienced recurring cycles of decline and recovery.

It reached its greatest extent under the reign of Justinian I (r. 527–565), who briefly reconquered much of Italy and the western Mediterranean coast. A plague began around 541, and a devastating war with Persia drained the empire's resources. The Arab conquests led to the loss of the empire's richest provinces—Egypt and Syria—to the Rashidun Caliphate. In 698, Africa was lost to the Umayyad Caliphate, but the empire stabilised under the Isaurian dynasty. It expanded once more under the Macedonian dynasty, experiencing a two-century-long renaissance. Thereafter, periods of civil war and Seljuk incursion resulted in the loss of most of Asia Minor. The empire recovered during the Komnenian restoration, and Constantinople remained the largest and wealthiest city in Europe until the 13th century.

The empire was largely dismantled in 1204, following the sack of Constantinople during the Fourth Crusade; its former territories were then divided into competing Greek rump states and Latin realms. Despite the eventual recovery of Constantinople in 1261, the reconstituted empire wielded only regional power during its final two centuries. Its remaining territories were progressively annexed by the Ottomans in a series of wars fought in the 14th and 15th centuries. The fall of Constantinople to the Ottomans in 1453 brought the empire to an end, but its history and legacy remain topics of study and debate to this day.