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
Delian League to the Persian Empire, and Eretria was duly destroyed. However, the invasion ended in 490 BC with the decisive Athenian victory at the Battle of Marathon. Between the two invasions, Darius died, and responsibility for the war passed to his son Xerxes I. Xerxes then personally led a second Persian invasion of Greece in 480 BC, taking an enormous (although oft-exaggerated) army and navy to Greece. Those Greeks who chose to resist (the 'Allies') were defeated in the twin simultaneous battles of Thermopylae on land and Artemisium at sea. All of Greece except the Peloponnesus thus having fallen into Persian