In the 4th century BC Phocis was constantly endangered by its Boeotian neighbours. After helping the Spartans to invade Boeotia during the Corinthian War (395-–94), the Phocians were placed on the defensive. They received assistance from Sparta in 380, but were afterwards compelled to submit to the growing power of Thebes. The Phocian levy took part in the inroads of Epaminondas into Peloponnesus, except in the final campaign of Mantinea (370–362), from which their contingent was withheld. In return for this negligence the Thebans fastened a religious quarrel upon their neighbours, and secured a penal decree against them from the Amphictyonic synod (356). The Phocians, led by two capable generals, Philomelus and Onomarchus, replied by seizing Delphi and using its riches to hire a mercenary army. With the help of these troops the Phocian League at first carried the war into Boeotia and Thessaly, and though driven out of the latter country by Philip of Macedon, maintained itself for ten years, until the exhaustion of the temple treasures and the treachery of its leaders placed it at Philip's mercy.

A Smaller History of Greece by William Smith:

Another war, which had been raging during the same time, tended still further to exhaust the Grecian states, and thus pave the way for Philip's progress to the supremacy. This was the SACRED WAR, which broke out between Thebes and Phocis in the same year as the Social War (B.C. 357). An ill-feeling had long subsisted between those two countries. The Thebans now availed themselves of the influence which they possessed in the Amphictyonic council to take vengeance upon the Phocians and accordingly induced this body to impose a heavy fine upon the latter people, because they had cultivated a portion of the Cirrhaean plain, which had been consecrated to the Delphian god, and was to lie waste for ever.

The Phocians pleaded that the payment of the fine would ruin them; but instead of listening to their remonstrances, the Amphictyons doubled the amount, and threatened, in case of their continued refusal to reduce them to the condition of serfs. Thus driven to desperation, the Phocians resolved to complete the sacrilege with which they had been branded, by seizing the very temple of Delphi itself. The leader and counsellor of this enterprise was Philomelus, who, with a force of no more than 2000 men, surprised and took Delphi. At first, however, he carefully abstained from touching the sacred treasure; but being hard pressed by the Thebans and their allies, he threw off the scruples which he had hitherto assumed, and announced that the sacred treasures should be converted into a fund for the payment of mercenaries. On the death of Philomelus, who fell in battle, the command was assumed by his brother Onomarchus, who carried on the war with vigour and success. But he was checked in his career by Philip, who had previously been extending his dominion over Thessaly, and who now assumed the character of a champion of the Delphic god, and made his soldiers wear wreaths of laurel plucked in the groves of Tempe. He penetrated into Thessaly, and encountered the Phocians near the gulf of Pagassae. In the battle which ensued, Onomarchus was slain, and his army totally defeated (B.C. 352). This victory made Philip master of Thessaly. He now directed his march southwards with the view of subduing the Phocians; but upon reaching Thermopylae he found the pass guarded by a strong Athenian force, and was compelled, or considered it more prudent, to retreat.