The ancient Greek city of Byzantion was established by a group of colonists from Megara in the seventh century B.C. According to the legend, the site owed its name to its celebrated founder, Byzas. Byzantion was founded on the first hill of the European shore of the Bosporus, and, from the beginning, its inhabitants enjoyed access to a natural and safe harbor on the Golden Horn. Following the fate of other Greek cities in Asia Minor, Byzantion was conquered by the Persians in 546 B.C., but in 479 B.C. it was liberated by the fleet of the Spartan-led Greek alliance, and subsequently came under the control of Athenian leadership (Delian League). Despite some short periods of Peloponnesian influence, the city of Byzantion remained a member of the Athenian alliance for much of the fifth century B.C. Byzantion was amongst the first recorded city members of the Second Athenian Confederation established in early 378 B.C. After two persistent sieges by the army of Philip ΙΙ of Macedon, Byzantion was forced to enter an alliance with him in c. 339 B.C. The city retained an autonomous status throughout the Hellenistic period, and was actively involved in a war against Rhodes in 220 B.C. over the imposition of tolls on the Bosporus Straights, which eventually ended in a peace treaty.      

The rapid Roman expansion and the unsettling conditions in Asia Minor toward the end of the Hellenistic era forced many kingdoms and cities, including Byzantion, to ally with the new world power, Rome, in c. 133 B.C. The Romans granted freedom and privileges to many regions in the Greek-speaking East, including Byzantion, but these were temporarily revoked by the emperor Vespasian in c. 70 A.D. during his administrative re-organization of the empire’s provinces. The political ferment following Emperor Commodus’s death on the last day of 192 A.D. brought a crisis to Byzantion; the city was besieged by the armies of the then eparch of Pannonia, Lucius Septimius Severus (r. 193-201 A.D.), when the civic authorities expressed their support for his opponent, Pescennius Niger. After a major siege lasting from 193 to 195 A.D. Byzantion surrendered to the Roman army, who then destroyed substantial parts of the city. However, Severus’s son and successor, Caracalla, convinced his father of the city’s strategic importance. Septemius Severus went on to restore the ruined walls and, among other monuments, provided Byzantion with a new colonnaded avenue, a theater, a large hippodrome, and the famous baths of Zeuxippos. Stability was maintained until around 257 A.D. when the city experienced the invasion of the Goths, who had been raiding wealthy urban sites in the southern Balkans since 250. Although Perinthus became the capital of the province of Thrace under Emperor Diocletian (284-305 A.D.) at the expense of Byzantion, the latter was the stage of a major victory of Constantine I over his long-time rival, Licinius, in 324. As a commemorative act, Constantine renamed Byzantion and founded Constantinople, the New Rome.