The siege of Olynthus (348 BC) saw Philip II of Macedon complete his conquest of the Chalcidic League, one of his more powerful immediate neighbours, and an ally for several years.
At the start of his reign Philip had agreed an alliance with Athens, and protected by this alliance had dealt with problems on his northern and western borders. However in 357 BC, after the outbreak of the Social War (357-355 BC), a revolt against Athens by some members of her League, he besieged and captured Amphipolis (357 BC), a city that Athens had founded, but soon lost. This triggered a war with Athens, and so Philip needed a new ally on the Aegean coast. He won over Olynthus and the Clalcidic League by promising to capture Potidaea, a former league member then held by Athens. Olynthus accepted the offer and agreed to an alliance. Potidaea fell in 356 BC and was restored to the League.
Over the next few years Philip's power in Thrace greatly increased. Olynthus began to make overtures towards Athens, a breach of the spirit of the alliance with Philip. Late in 351, while returning from the successful siege of Heraeum, Philip marched his army through Chalcidice, perhaps in an attempt to intimidate his allies. This didn't work. Olynthus gave refuge to Philip's half brothers Arrhidaeus and Menelaus, perhaps as part of plan to replace Philip on the Macedonian throne.
In the third quarter of 349 BC Philip invaded League territory. He besieged and captured the otherwise unknown town of Zeira (or Zereia), and after its fall destroyed it. The League began to fall apart as its members surrendered to Philip to avoid the same fate. Stagirus, the birthplace of Aristotle, was amongst the places taken razed to the ground.
Olynthus sent envoys to Athens to beg for assistance, and under pressure from Demosthenes (this was when he produced his First Olynthiac Oration) the Athenian Assembly dispatched a token force north. This first contribution consisted of 2,000 peltasts and thirty triremes commanded by Chares. This first period of command ended when Chares was recalled to Athens and prosecuted for misconduct.
Olynthus was given some breathing room by affairs in Thessaly. Philip had expelled the tyrants of Pherae, but one of them, Peitholaus, had managed to regain control of the city. This was more important than the campaign against Olynthus, and so Philip left for Thessaly and expelled Peitholaus once again.
This gave the Athenians time to dispatch a second force, consisting of 4,000 peltasts, 150 cavalry and 18 triremes, commanded by Charidemus. He cooperated with the Olynthians and advanced into Pallene (the westernmost peninsula of Chalcide) and Bottiaea, ravaging the country. Part of this raid recovered areas that had already fallen to Philip, forcing him to reconquer them,
Probably in March 348 Philip resumed his campaign against Olynthus. Her port, at Mecyberna, was captured, as was the nearby town of Torone. Olynthus was then besieged, although the defenders did manage to get another message through to Athens.
Athens sent a third force in response to this embassy from Olynthus, calling for a force of Athenian citizens. A force of 2,000 hoplites, 300 cavalry and 17 triremes, commanded by Chares, who had now been pardoned, was dispatched, but it probably arrived too late, delayed once again by the Etesian Wind, a seasonal north wind.
Diodorus gives a very brief account of the siege. Philip captured Mecyberna (the port of Olynthus) and Torone (at the tip of Sithonia, the middle of the three peninsulas of Chalcidice) with the aid of traitors within the city. He then defeated the Olynthians in two battles, and forced them back into the city. He made a series of assault on the walls in which he lost many men. The city was finally betrayed by Euthycrates and Lasthenes, the chief magistrates of Olynthus, who accepted bribes from Philip. According to Demosthenes the two men didn’t benefit from their actions, and instead 'met the most ignominious fate of all'. He also gives some details of the bribes - Lasthenes roofed his house with a gift of Macedonian timber, Euthycrates received a large herd of cattle. Philip was known to have said that he expanded his kingdom more with gold than with his army, and this was a good example of that trend.
One of these two battles might have involved the incident mentioned by Demosthenes (On the False Embassy, Dem.19 267) in which 500 cavalry with all of their equipment were betrayed by their officers and captured by the Macedonians.
The city fell in the autumn of 348 BC. Any captured Athenians were imprisoned, while the locals were sold into slavery. Chalcidice became part of Macedonia. Some of the money taken in loot here was used to bribe potential allies in other Greek cities. Arrhidaeus and Menelaus, who had remained in the city, were captured and killed.
Demosthenes gives us overall totals for the Athenian contribution to the war - 10,000 mercenaries, 4,000 citizen soldiers and 50 war galleys. He also gives the Olynthians 10,000 men with 1,000 cavalry. He also claimed that 32 towns in Chalcidice were wiped out.
Even as this siege was coming to its end, Philip began to hint that he wanted peace with Athens. This eventually led to the Peace of Philocrates (346 BC), the agreement that ended the ten year long 'War of Amphipolis' between Athens and Philip, and helped clear the ground for Philip to end the Third Sacred War.