Byzantium was an ancient Greek city on the European side of the Bosporus, the strait that separates Europe from Asia. The city became the capital of the Roman Empire under Constantine I in the second decade of the fourth century. Following his death in 337, the name of the city was changed to Constantinople in his honor.
The origins of Byzantium are shrouded in legend. Byzantion, the city's original name before it took its Latinized form of Byzantium, was said to have been founded by Greek colonists from Megara in 667 BC and named after their king Byzas or Byzantas (in Greek: Βύζας or Βύζαντας). Byzas was said to have consulted the Oracle at Delphi to ask where should he establish his new city.The Oracle told him it would be found "opposite the blind." At the time, he did not know what this meant. But when he came to the Bosporus he realized what it meant. On the eastern, Asian shore of the Bosporus was a Greek city, Chalcedon, whose inhabitants had not seen the location across the strait with a fine harbor, later to be called the Golden Horn. Here Byzas established his new colony, naming it after him.
Byzantium became a leading center of trade, positioned as it was on the only water entrance to the Black Sea. It was also a strategic place off the Aegean Sea and in the path of warring forces traveling from Asia Minor to the European lands. In the fifth century BC, Byzantium was destroyed by the Persian forces of Darius I and rebuilt by the Spartans. Over the following centuries the city came under alternate control of Athens and Sparta before falling to Alexander of Macedonia. Later, it was attacked by Scythians, Celts, and other warring groups until the city slowly came under Roman dominance.
During the Roman civil war in the last decade of the second century AD, Byzantium suffered serious damage that included the razing of its walls. The city was rebuilt by emperor Septimius Severus and soon regained its prosperity. In 324, the Roman emperor of the West, Constantine I, defeated emperor Licinus of the East near Byzantium. Now as the sole emperor of the Roman empire, and attracted by the city, Constantine soon moved his capital to the Byzantium.
Under Orthodox Christian tradition, Byzantium was evangelized by the Apostle Andrew and subsequently became the see of a bishop subordinate to the Bishop of Heraclea along with a number of other suffragan sees. With the establishment of Byzantium as Constantine's new capital, a move that was recognized at the First Ecumenical Council at Nicea as the New Rome (Nova Roma), the bishop of the city gained greater prerogatives as the emperor's bishop. The city became known as Constantinople after Constantine's death, and the seat of the Bishop of Constantinople, later the Patriarchs of Constantinople. With the ascension in rank of the Patriarch of Constantinople, the Bishop of Heraclea became the senior subordinate with the privilege of handing the crosier a newly elected Patriarch of Constantinople.