The siege of Byzantium (408 BC) was an Athenian victory that saw them regain control over the Bosphorus, and remove a threat to Athens's food supplies from the Black Sea (Great Peloponnesian War).
Byzantium had been part of the Athenian Empire, but it had rebelled after the Athenian disaster at Syracuse, and by 408 was held by a mixed garrison of Byzantines, Perioci (free non-citizens of Sparta), Neodamodes (Helots freed after serving in the Spartan army), Megarians and Boeotians, all commanded by the Spartan governor Clearchus.
The Athenian army, at least 5,000 strong, was commanded by Theramenes, Thrasyllus and Alcibiades. In the spring of 408 they had besieged Chalcedon, on the opposite side of the Bosphorus, and had won a victory outside the city. Theramenes had then come to an agreement with the Persian satrap Pharnabazus in which the Athenians agreed to halt military activities against Chalcedon in return for a payment of 20 talents, as well as the regular tribute from Chalcedon and free passage for an embassy to visit the Persian emperor.
The main focus of the Athenian military then moved across the water to Byzantium. The Athenians built siege lines around the city, cutting it off by land and sea. They also positioned sharpshooters around the city, and carried out a number of unsuccessful assaults on the walls.
The turning point of the siege came when Clearchus decided to leave the city to collect money from Pharnabazus and to raise a fleet to harass the Athenians at sea. In his absence a number of Byzantines, angered by the way most food in the city was going to the Peloponnesian troops, decided to surrender the city to the Athenians.
Our two main sources give different accounts of the surrender. According to Xenophon the Byzantine plotters simply let the Athenians into the city. The garrison found them already in occupation of the centre of the city, and had no choice but to surrender.
Diodorus Siculus gives a rather more complex account. In order to convince the garrison that they were abandoning the siege, the Athenian fleet put to sea and their army withdrew, but under cover of darkness their fleet launched a noisy attack on the harbour area, while the army hid close to the walls. The garrison rushed to the harbour to fight off the naval attack, at which point the Athenian supporters inside the city signalled to the besieging army, which entered the city using ladders. Half of the garrison rushed back to the walls, and a fierce battle broke out. The battle only ended after Alcibiades announced that no harm would be done to the Byzantines, at which point they changed sides.
In both accounts the captured prisoners were taken back to Athens. Byzantium was treated generously, and was re-admitted to the Empire as allies. In the aftermath of this victory Alcibiades was finally able to return to Athens after years in exile, although his return to favour would be short lived, and in 407 he would return to exile.