Expansion of Macedonia under Philip II
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 ostensibly 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
In 353 BC, Onomarchos took advantage of the fact that Thebes, financially exhausted, sent out a troop of 5,000 Theban soldiers as mercenaries to support the revolt of Artabazus, satrap of Hellespontine Phrygia, against the Persian king. He led an attack against Locris and captured Thronion, which constituted a key strategic point on the route network of central mainland Greece. He turned south and invaded Doris and eventually Boeotia, where he was finally controlled by the allied Boeotians close to Chaeronea.