Solidus with the image of Justinian the Great
(527-565) (see Byzantine insignia)
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 Roman Empire (Ancient Greek: Βασιλεία Ῥωμαίων, tr. Basileia Rhōmaiōn; Latin: Imperium Romanum), and Romania (Ῥωμανία).
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 Nova Roma ("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 significantly 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 the 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 Macedonian dynasty (10th-11th centuries), the Empire again expanded and experienced a two-century long renaissance, which came to an end with the loss of much of Asia Minor to the Seljuk Turks after the Battle of Manzikert in 1071. This battle opened the way for the Turks to settle in Anatolia as a homeland.