The Byzantine Empire was the predominantly Greek-speaking continuation of the Roman Empire during Late Antiquity and the Middle Ages. Its capital city was Constantinople (modern-day Istanbul), originally known as Byzantium. Initially the eastern half of the Roman Empire (often called the Eastern Roman Empire in this context), it survived the 5th century fragmentation and collapse of the Western Roman Empire and continued to thrive, existing for an additional thousand years until it fell to the Ottoman Turks in 1453. During most of its existence, the empire was the most powerful economic, cultural, and military force in Europe. Both "Byzantine Empire" and "Eastern Roman Empire" are historiographical terms applied in later centuries; its citizens continued to refer to their empire as the "Empire of the Romans" (Greek: Βασιλεία Ῥωμαίων, translit. Basileia Rhōmaiōn), and "Land of the Romans" (Greek: Ῥωμανία, translit. Rhōmania).
Several events from the 4th to 6th centuries mark the transitional period during which the Roman Empire's east and west divided. In 285, the emperor Diocletian (r. 284–305) partitioned the Roman Empire's administration into eastern and western halves. Between 324 and 330, Constantine I (r. 306–337) transferred the main capital from Rome to Byzantium, later known as Constantinople ("City of Constantine") and New Rome. Under Theodosius I (r. 379–395), Christianity became the Empire's official state religion and others such as Roman polytheism were proscribed. And finally, under the reign of Heraclius (r. 610–641), the Empire's military and administration were restructured and adopted Greek for official use instead of Latin. In summation, Byzantium is distinguished from ancient Rome proper insofar as it was oriented towards Greek rather than Latin culture, and characterised by Orthodox Christianity rather than Roman polytheism.
The borders of the Empire evolved a great deal over its existence, as it went through several cycles of decline and recovery. During the reign of Justinian I (r. 527–565), the Empire reached its greatest extent after reconquering much of the historically Roman western Mediterranean coast, including north Africa, Italy, and Rome itself, which it held for two more centuries. During the reign of Maurice (r. 582–602), the Empire's eastern frontier was expanded and north stabilised. However, his assassination caused a two-decade-long war with Sassanid Persia which exhausted the Empire's resources and contributed to major territorial losses during the Muslim conquests of the 7th century. During the 10th-century Macedonian dynasty, the Empire experienced a golden age, which culminated in the reign of Emperor Basil II "the Bulgar-Slayer" (r. 976–1025). However, shortly after Basil's death, a neglect of the vast military built up during the Late Macedonian dynasty caused the Empire to begin to lose territory in Asia Minor to the Seljuk Turks. Emperor Romanos IV Diogenes (r. 1068–1071) and several of his predecessors had attempted to rid Eastern Anatolia of the Turkish menace, but this endeavor proved ultimately untenable - especially after the disastrous Battle of Manzikert in 1071.
Despite a prominent period of revival (1081–1180)
under the steady leadership of the Komnenos family
, who played an instrumental role in the First
and Second Crusades
, the final centuries of the Empire exhibit a general trend of decline. In 1204, after a period of strife following the downfall of the Komnenos dynasty, the Empire was delivered a mortal blow by the forces of the Fourth Crusade
, when Constantinople was sacked
and the Empire dissolved and divided
into competing Byzantine Greek and Latin realms
. Despite the eventual recovery of Constantinople and re-establishment of the Empire in 1261
, Byzantium remained only one of a number of small rival states in the area for the final two centuries of its existence. This volatile period led to its progressive annexation by the Ottomans
over the 15th century and the Fall of Constantinople