Byzantium, city of antiquity, founded about 660 bc by Greek colonists. It was situated on the European side of the Bosporus and occupied part of the site of present-day İstanbul. With an excellent harbor, later called the Golden Horn, a strategic location between the Black and Mediterranean seas, and abundant fisheries, Byzantium soon became a leading port and distribution center, especially noted for its grain trade. Troops of the Persian ruler Darius I destroyed Byzantium early in the 5th century bc. In 479 bc it was rebuilt by the Spartans. During the next 100 years Athens repeatedly disputed Spartan control of the city. The Athenian general Alcibiades captured it in 409 bc. Four years later the Athenians were expelled by an expedition under the Spartan general Lysander. Byzantium fell to Athens again in 390 bc, but, in alliance (357 bc) with the Greek islands of Khíos, Rhodes, and Kos, as well as Kariá, a Persian satrapy, the city waged a successful struggle for freedom. In 341-340 bc Athens rallied to defend the city, then under siege by Philip II of Macedonia. During the reign (336-323 bc) of Philip's son Alexander the Great, Byzantium was forced to recognize Macedonian suzerainty; under his successors the Byzantines regained independence. The city was subsequently attacked by the Scythians, and in 279 bc the Celts, having overrun neighboring Thrace, imposed tribute on the city. To secure funds, the Byzantines levied a toll on ships passing through the Bosporus, provoking war with Rhodes.

The Byzantines supported Rome in the war (191-188 bc) against the Syrian king Antiochus III. In appreciation of this help and additional aid extended during the Mithridatic Wars (88-65 bc), the Romans recognized Byzantium as a free confederate city. It was subsequently subjected to Roman imperial control, however, and required to pay tribute until the reign (ad 41-54) of Emperor Claudius I. In the civil war (194) between the Emperor Lucius Septimius Severus and the military leader Pescennius Niger, Byzantium sided with Niger. Severus captured Byzantium in 196, revoked its privileges, and razed its walls. The emperor of the West, Constantine the Great, defeated the Roman emperor Licinius (ad270?-325) near Byzantium in 324 and soon thereafter began rebuilding the city as his new capital, a new Rome (Nova Roma) in 330. Byzantium was subsequently renamed Constantinople (present-day İstanbul). Capital of the Byzantine Empire until its fall in 1453, the city, with a population of about 1 million was by far the largest and richest city in Christendom during the Middle Ages.