Battle of Crocus Field
The Third Sacred War (often just called 'the' Sacred War) broke out in 356 BC, and would present Philip with his first real opportunity to expand his influence into the affairs of central and southern Greece. The war was caused by the refusal of the Phocian Confederation to pay a fine imposed on them in 357 BC by the Amphictyonic League, a pan-Greek religious organisation which governed the most sacred site in Ancient Greece, the Temple of Apollo at Delphi. Behind the religious element, there probably lay a display of "realpolitik" in bringing charges against the Phocians, instigated by the Thebans. At this time, Thebes controlled a majority of the votes in the council, and at the autumn meeting in 357 BC, the Thebans were able to have both the Phocians (for the cultivation of the sacred land) and the Spartans (for occupying Thebes some 25 years previously) denounced and fined. Since the fines for both parties were "unjustifiably harsh", the Thebans probably expected neither party to pay, and thus to be able to declare a "sacred war" on either.
Third Sacred War
Now that he was in control of Thermopylae, Philip could be certain of dictating the terms of the end of the Sacred War, since he could now use force against any state that did not accept his arbitration. He began by making a truce with Phalaikos on 19 July; Phalaikos surrendered Phocis to him, in return for being allowed to leave with his mercenaries and go wherever he wished. Cawkwell suggests that Phalaikos probably collaborated with Philip in 346 BC, allowing Philip to take Thermopylae in return for lenience for him and his men. Otherwise, it is difficult to see how Philip could have advertised his campaign so far in advance (and been so confident of success), and yet not been stopped at Thermopylae. Philip restored to Boeotia the cities that Phocis had captured during the war (Orchomenos, Coroneia and Corsiae), and then declared that the fate of Phocis would not be decided by him, but by the Amphictyonic Council. This caused great panic in Athens, since the Phocians could never hope for mercy from the Amphictyons, and since Athens had also (having allied with Phocis) shared in the same sacrilege. However, it is clear that Philip was dictating the terms behind the scenes; allowing the Amphictyons the formal responsibility allowed him to dissociate himself from the terms in the future.