When Philip had entirely subdued the Thracians and Olynthians, he helped the Thessalians to get rid of their tyrant; and, adding their cavalry to his infantry, he boasted of as fine an army as the Greeks had ever been able to muster. He was very anxious to find a pretext to march into Greece at the head of this force, because he thought that, once there, he would soon manage to become master of all the towns. And the excuse for which he longed so much soon came.

A contest known as the Sacred War was going on in Greece at that time. It had arisen because the Phocians had taken possession of lands that were left waste in honor of the god Apollo. The Amphictyonic Council said they should pay a fine for this offense; and the Phocians, angry at being thus publicly reproved, defied the council. To show how little they intended to obey, they not only kept the land they had taken, but robbed the temple at Delphi. Then they used the money thus obtained to win over some allies, and soon began to make war against the people who obeyed the council.