The siege of Potidaea (356 BC) saw Philip II of Macedon capture the strongly fortified city at the head of the Pallene peninsula, but then hand it over to Olynthus in order to secure an alliance with that city.
Potidaea was located at the narrow neck of the Pallene peninsula, the westernmost of the three peninsulas of Chalcidice. She had been a member of the Chalcidian League, led by nearby Olynthus, but in around 363-361 BC Athens took control of the city.
At the start of his reign Philip II of Macedon had formed an alliance with Athens, but this was ended by his successful capture of Amphipolis (357 BC). This city, to the east of Chalcidice, had been founded by Athens, but quickly lost to Sparta. Regaining control of Amphipolis had been a long term Athenian objective, and after the Macedonians took the city the Athenians declared war, starting the ten year long 'War of Amphipolis'. At this point Athens was distracted by the Social War, a conflict with her own allies, and was unable to intervene effectively in the north.
Having alienated the Athenians, Philip decided to form an alliance with the Chalcidian League. At this point the league was in negotiations with the Illyrian King Grabus, and the Olynthians had also attempted to negotiate an alliance with Athens. They were rebuffed by the Athenians. Philip took advantage of this by making them a generous offer. He would hand over the border territory of Anthemus, and help the Olynthians capture Potidaea. In return the Olynthians agreed not to enter into an alliance with Athens without Philip.
The siege began around July 356 BC, just as the Etesian Wind began to blow. This northerly wind made it harder for Athens to send any fleets into the northern Aegean, so reduced the chance of any interference in the siege. It is possible that Athens did indeed try and send aid, but if so it arrived too late.
Potidaea surrendered to Philip in the autumn of 356 BC. Any Athenians found in the city were sent home, but the Potidaeans themselves were sold into slavery. The city and the surrounding area were then handed over to Olynthus and the Chalcidic League.
Plutarch records that Philip received three items of good news just after the fall of the city - the victory of his horse at the Olympics, Parmenion's victory over Grabus and the birth of the future Alexander the Great. This story doesn’t entirely work, as Alexander is said to have been born in July 356 and the Olympics were held in August or September, but it does demonstrate how much progress Philip had made in the first few months of his reign.
Olynthus didn’t enjoy its success for too long. Their relationship with Philip soon soured, and the city was besieged and destroyed in 348 BC.