Amphictyonic Leagues [from amphictyones, 'dwellers around'] were associations of communities who lived in the neighbourhood of ancient Greek religious sites. They supported temples or sacred places, kept religious festivals and conducted events including the Olympic and Pythian Games.
The most important, such as that of Delphi, came to include representatives from much of Greece. The Delphic league administered the temple of Apollo and conducted the Pythian games.
With twelve delegates, the longest-standing Amphictyonic League protected and administered the temple of Apollo in Delphi and the temple of Demeter in Anthela. The council also had the power to impose sentences on those who violated sanctuaries ranging from fines to expulsion from the League.
Disputes with the League over matters relating to the temples resulted in four Sacred Wars waged to protect Apollo's shrine and punish states supposedly guilty of sacrilege.
The First (596 to 585 BCE, a.k.a. the Cirrhean War) followed a dispute about access to the Delphic Oracle after it became independent from the city of Crissa, to which the temple had belonged. A decade of fighting saw the city destroyed and its fertile plain dedicated to Apollo, Leto, Artemis, and Athena Pronaia. The area became the venue for the Pythian Games, held every four years under the direction of the League, and was not to be cultivated.
The Second (449-448) followed Phocian attempts to control the sanctuary. An intervention by Sparta restored the status quo. However, once the Spartans withdrew, the Athenians restored Delphi and the management of the Pythian Games to Phocis.
Under the Peace of Nicias between Athens and Sparta, Delphi became autonomous again in 421. The Phocians had probably already lost control of the area after the Boeotian League defeated Athens at the Battle of Coronea in 447.
The Third Sacred War (356–346) followed another Phocian attack on the sanctuary. Under Philomelos, the Phocians captured and sacked Delphi. At the League's meeting in the spring of 356, the Thebans had denounced Phocis and Sparta for not paying fines. The Phocians had allegedly been cultivating the Crisaean plain. Sparta had despoiled sanctuaries when they seized the Theban Cadmea in 382.
After the Phocians refused to pay and seized Delphi, Sparta's financial support allowed Philomenus to hire five thousand mercenaries and thwart attempts to dislodge him from the sacred site.
Although most of the Greek states became involved, the war was indecisive until Philip II of Macedon intervened and destroyed Phocis. In 346, after the Peace of Philocrates between Athens and Macedon, the Phocians were expelled from the League. Their two votes went to Philip, giving him another avenue to further his expansionist policy in Greece.
Athenian and Amphissan accusations and counter-accusations of sacrilegious actions resulted in a Fourth Sacred War (339–338). Philip II's appointment as commander of the League's army provided him with a pretext to invade Greece. Philip occupied Elatea in Phocis and then turned on Thebes, which was now allied with Athens. Philip's victory in the Battle of Chaeronea (338 BCE) gave him the hegemony of Greece.
Sources:
Simon Hornblower, Antony Spawforth, and Esther Eidinow (eds.), The Oxford Classical Dictionary
M. C. Howatson (ed.), The Oxford Companion to Classical Literature
John Roberts (ed.), Oxford Dictionary of the Classical World,
Wikipedia