The siege of Byzantium (340-339 BC) was an unsuccessful attempt by Philip II to defeat a former ally, and was begun after his siege of nearby Perinthus ran into difficulties. Both sieges came in the build-up to the Fourth Sacred War.
In around 343 BC a new Athenian commander, Diopeithes, was sent to the Chersonese, to support the Athenian colonists in the area. He soon became involved in a clash with Cardia, an ally of Philip's at the northern end of the Chersonese. During this struggle Diopeithes rather overstepped the mark, capturing two Macedonian heralds, and torturing and ransoming the second. Unsurprisingly Philip decided to move to the area to try and support his allies, and he called for help from Perinthus and Byzantium. Both cities were allied with Philip, but he must have seemed a rather more threatening presence since expanding his empire across most of Thrace, and both refused to help.
Philip responded by besieging Perinthus (340-339 BC). Byzantium sent supplies and men to the besieged city, as did the Persians. Philip's siege engines managed to break down the outer city wall, only to find that the defenders had built a new wall between the first rows of houses. Philip was also struggling at sea. At this stage he was officially at peace with Athens, but the Athenians had prevented his fleet from passing through the Hellespont. Eventually he decided to land his troops on the Chersonese, and use them to escort the fleet through Athenian territory. Even this didn't help, and after about three months Philip decided to launch a surprise attack on Byzantium.
In the late summer of 340 BC he led half of his army to Byzantium, but the city turned down his offer of terms, and prepared to resist. Most of their men and weapons were indeed at Perinthus, but the defenders managed to survive the initial crisis.
At about the same time an Athenian grain fleet was gathering at Hieron, waiting for Chares to escort it to the Aegean. While Chares was away meeting with local Persian officials, Philip's fleet captured the merchant fleet. Fifty neutral ships were released, but 180 Athenian ships were captured. The supplies went to Philip's armies outside Byzantium and Perinthus while the timber from the ships was used to build more siege engines. Philip then sent a letter to Athens in which he claimed that the merchant ships had been supplying his enemies. This letter was treated as a declaration of war in Athens, and open conflict between the two finally began.
Chares was ordered to use his forty ships to relief Byzantium. The Macedonian fleet was forced to retreat into the Black Sea, where it could neither help at the siege, nor return to Macedon. Byzantium received help from her allies at Chios, Cos and Rhodes, although the Persians don't appear to have intervened here, despite having helped the defenders of Perinthus.
At first the relationship between the Byzantines and the Athenians wasn't good, as the Byzantines didn't trust Chares. Things got better when a second fleet, commanded by Phocion and Cephisophon, reached the area. Phocion and the Byzantine commander Leon were personal friends, and they were able to coordinate a successful defence. The Byzantines also received aid from their allies at Chios, Cos and Rhodes.
In the early spring of 339 BC Philip launched one last assault on the walls, using the spring moonlight to aid a night attack. The barking of dogs was said to have betrayed the attack, and Philip decided to give up and retreat. His biggest problem was that his fleet was trapped in the Black Sea by an Athenian force that held the Bosporus.
Philip resorted to a simple trick to get his fleet to safety. He sent a letter to Antipater, informing him that Thrace was in revolt, and his garrisons under siege. Antipater was ordered to join Philip as he marched into Thrace to restore control. The Athenians relaxed their guard in the Bosporus, or possibly withdrew their fleet from the area, allowing Philip to get his own fleet out of the Black Sea.
In both sieges Philip had the support of the great Thessalian siege engineer Polyeidus, and had the most modern siege engines. His failure demonstrated how hard it was to capture coastal cities if you didn’t also have control of the seas.
Byzantium was soon forced to come to terms to Philip, probably after the Athenians and Thebans suffered their great defeat at the battle of Chaeronea (338 BC), although this might have come earlier. She retained enough independence to continue to issue her own coinage.