The siege of Zeira (349 BC) came at the start of Philip II of Macedon's campaign against Olynthus and Chalcidice, and saw him capture and destroy the city.
Early in his reign Philip had formed an alliance with the Chalcidic League, and in 356 BC had even captured the city of Potidaea and handed it over to the League. Over the next few years his relationship with the league slowly worsened. Olynthus began to make overtures towards Athens, although didn't form an alliance, as that would have breached the terms of the alliance with Philip. In 351, on his way back from the siege of Heraeum, Philip marched through League territory, in an attempt to intimidate Olynthus. This effort failed, and may even have encouraged Olynthus to give refuge to Philip's half brothers Arrhidaeus and Menelaus.
War between Philip and the League began in 347 BC. He crossed the border, and attacked the fortress of Zereia or Zeira. This place is otherwise unknown and it probably didn't delay Philip for long. The fortress was besieged and destroyed. In the aftermath of this success other nearby towns surrendered to Philip in order to avoid sharing the same fate. It is possible that Zereia was in Pallene, as this area was raided by the second Athenian expedition sent to help Olynthus.
Philip was probably planning to attack Olynthus itself in the same year, but he was distracted by trouble in Thessaly. He had previously expelled the Tyrants of Pherae, but one of them, Peitholaus, appears to have regained control of the city. This forced Philip to leave Chalcidice to campaign in Thessaly, where he quickly expelled Peitholaus once again.
This was only a temporary reprieve for Olynthus, although it did make Philip's task harder. By the time he returned to besiege Olynthus in 348 BC the Athenians were ready to send help, but even their three expeditions were unable to save the city.