The siege of Amphipolis (357 BC) was an early victory for Philip II of Macedon, and saw him capture a key foothold in Thrace, although at the cost of permanently damaging his relationship with Athens.
Amphipolis was an important city just inland from the coast, to the east of Chalcidice. It had been founded by the Athenians in an attempt to control the Thracian coast, and the trade routes into the Black Sea. Possession of Amphipolis was a long-term Athenian aim, although the city had only been in their hands for a short time (between the city's foundation in 437 BC and its loss to the Spartans in 424 BC (followed by the Athenian defeat at the battle of Amphipolis in 422 BC)). In 359 Athens had agreed an alliance with Philip II, perhaps in the belief that he had promised to support their claim to the city.
According to Demosthenes Philip had promised to hand Amphipolis over to Athens once he had taken in, in return for Pydna, a Macedonian port that had been in Athenian hands since c.364 BC. However this was a secret treaty,
This alliance freed Philip to concentrate on his northern and western borders. First he defeated the Paeonian tribes, to the north of Macedon, and then in 358 he defeated King Bardylis of Illyria (battle of the Erigon Valley or Lyncus Plain). These victories pushed the Macedonian frontier further away from the Macedonian heartland.
In 357 BC Athens was weakened by the outbreak of the Social War (357-355 BC), triggered by a series of revolts against Athenian rule. Soon afterwards Philip attacked Amphipolis, either taking advantage of the Athenian weakness, or because that weakness reduced the value of the Athenian alliance. His pretext was that the people of Amphipolis were ill disposed towards him.
Diodorus gives us a brief account of the siege (Diodorus 16.8.2). Philip brought up his siege engines, and used battering rams in a severe and continuous assault on the wall. He soon created a breach in the wall and his troops stormed the city. After the fall of the city he exiled those who opposed him, but treated the rest of the inhabitants leniently. Demosthenes tells us that two envoys from Amphipolis (Hierax and Stratocles) reached Athens during 357 BC and asked the Athenians to take over their city,
Philip's success worried Olynthus, the leader of the Chalcidic League. Olynthus made overtures to Athens, but without success. They also entered into negotiations with Grabus, an Illyrian king. Philip responded with a counter offer. He promised to help restore League control of Potidaea, which had been an Athenian cleruchy since 361 and hand over the border territory of Anthemus. The Olynthians accepted Philip's offer, and agreed an alliance with him. One of the terms was an agreement not to enter into an alliance with Athens without Philip.
Soon after taking Amphipolis, Philip further expanded his power in the area. The Thracian king Cersobleptes attempted to capture the mining centre of Crenides, around the Pangaean Mountain. This was a centre of gold production, and so when the Crenideans appealed to Philip he was happy to help. Cersobleptes was forced back, and the scattered communities of Crenides were concentrated in one city, with the new name of Philippi.
Soon after these events Philip captured Pydna, at least according to Demosthenes with the help of traitors within the city.
Athens responded to the fall of Amphipolis by declaring war on Philip, triggering the ten year long 'War of Amphipolis'. Over the next decade the Athenians often planned to help Philip's enemies, but their forces almost always arrived too late to help, and the war was eventually ended by the Peace of Philocrates (346 BC), the same agreement that led to the end of the Third Sacred War.