Phocis sent out ambassadors to justify its actions, but soon had to defend the shrine against the hoplites of another League state called Locris. The Locrians weren't happy and had declared war on Phocis. They appealed for help from the Boeotians, the Thessalians and other Amphictyons. While Sparta and Athens watched their ally Phocis with interest, Phocis plundered the treasuries of Delphi to fund a huge mercenary army. It was determined to hold on to Apollo's sacred shrine.
The Sacred War saw ten years of bloody fighting concentrated in Phocis, Locris and western Boeotia. Philomelus died in a bloody rout of Phocian hoplites in 355, and his successor Onomarchus rallied the Phocians, and doubled the number of mercenaries after more plundering of the holy treasuries. In 354 the Phocians enjoyed numerous victories. The Thessalians needed some help in fighting the city-state of Pherae and turned to the barbarian warlord King Philip of Macedon. Looking for a way to take Delphi for himself and gain control over all the Greeks, Philip agreed to help the Thessalians. He became the tagos (leader) of the Thessalian League. Onomarchus with his larger army and use of catapults twice faced the Macedonians in battle, and twice defeated Philip's forces. In 353 the victorious general also fought the Thebans successfully; but Philip wanted vengeance and began the siege of Pherae in Thessaly. The Phocians, led by Onomarchus, faced Philip of Macedon once again, but this time they were defeated and brave Onomarchus was himself slain. The 3,000 Phocian prisoners were executed by drowning as a punishment for plundering the Delphic treasuries.
King Philip took Pherae and became the undisputed overlord of Thessaly. He would have pressed home the Phocian defeat by marching south, but Athenian hoplites blocked the pass at Thermopylae. It was becoming clear to all of the city-states what Philip intended, and in the years of brutal and unrelenting warfare to come the Sacred War would undoubtedly provide an opportunity for the warlord to pounce on the Greeks.
The Athenians had already come to blows with Philip; the warlord had captured the northerly Athenian outpost of Amphipolis in 357. The city controlled gold and silver mines which were a great source of income for Athens. Now Philip uses these riches to fund his Macedonian war machine and his expansion into northern Greece. He's already faced Athenian forces who were dispatched to retake Amphipolis, as well as the phalanxes of Olynthus and the Chalcidian League. Olynthus was once a Macedonian ally that helped capture a number of Athenian possessions in the north. It then became a turn-coat friend of Athens and the betrayal of this Chalcidian ally has inspired a ferocious campaign of Macedonian vengeance. This campaign continues.
In 356 Philip's sorceress wife Olympia gave birth to a son, Alexander. Rumours abounded that the boy was not his, but Philip declared Alexander his heir and began to groom him for kingship. Other rumours circulate that the boy is cursed with horns and that his real father is either a god or a monster. In the same month as the birth of his son, Philip entered the Olympic Games and won a spectacular victory in the four-horse chariot race. This victory gave him a great deal of prestige amongst the Greeks. Philip the Warlord may be uncouth, but he celebrated all of the best Greek traditions.