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.
Battle of Crocus Field
The ancient sources for the Third Sacred War are scant, and generally lacking in firm chronological information. The main source for the period is Diodorus Siculus's "Bibliotheca historica", written in the 1st century BC, which is therefore very much a secondary source. Diodorus is often derided by modern historians for his style and inaccuracies, but he preserves many details of the ancient period found nowhere else. Diodorus worked primarily by epitomizing the works of other historians, omitting many details where they did not suit his purpose, which was to illustrate moral lessons from history; his account of the Third Sacred War therefore contains many gaps.