Byzantine Empire, historical term for the successor state to the Roman Empire in the East. Its capital was Constantinople (now Istanbul), founded by Constantine I in 330 at the site of the ancient Greek Byzantium. The heartlands of the empire were Asia Minor and the Balkans, but at its height it ruled southern Spain, Italy, Sicily, northern Africa, Egypt, Syria, Palestine, the Crimean coast, Cyprus, and the Aegean islands. Its religion was Eastern Orthodox Christianity. Byzantine missionaries carried Christianity to Russia, and Byzantine theologians were among the chief Church Fathers.
The Roman Empire was divided after the death of Theodosius I in 395. By about 500 the Western Empire had fallen, Germanic invaders occupying Italy, Spain, and northern Africa. In the East, however, Roman institutions continued. However, after about 600, Roman institutions were replaced by typically Byzantine (Greek) ones. From the late seventh century onward, the Byzantine Empire was in frequent conflict both with the West and with the rising Muslim civilization in the East. Much of the empire's territory was lost, first to the Arabs, later to the Turks. The empire finally collapsed in 1453, when the Ottoman Turks captured Constantinople.