Expansion of Macedonia under Philip II a significant problem, blocking the main route into Greece. However, a second route into central Greece was available. In 480 BC, during the Battle of Thermopylae, the Persian king Xerxes had sent his army via a mountain track (the "Anopea") to outflank the pass. From this track, on the western side of Mount Kallidromon, another road led off and descended in Phocis. In 480 BC, a thousand Phocian troops were stationed above Thermopylae to guard the road and prevent a Persian assault on Phocis (though they notably failed to prevent the Persians using "Anopea"). However, in 339 BC, the Greeks
Battle of Thermopylae (279 BC) his forces, who were then free to ravage the country. After these expeditions returned home, Brennus urged and persuaded them to mount a third united expedition against central Greece, led by himself and Acichorius. A Greek coalition made up of Aetolians, Boeotians, Athenians, Phocians, and other Greeks north of Corinth took up positions at the narrow pass of Thermopylae, on the east coast of central Greece. During the initial assault, Brennus' forces suffered heavy losses. Hence he decided to send a large force under Acichorius against Aetolia. The Aetolian detachment, as Brennus hoped, left Thermopylae to defend their homes. The