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
Thermopylae shoreline has also advanced over the centuries because of the sedimentary deposition. The level of the Malian Gulf was also significantly higher during prehistoric times and the Spercheios River was significantly shorter. Its shoreline advanced by up to 2 kilometers between 2500 BC and 480 BC but still has left several extremely narrow passages between the sea and the mountains. The narrowest point on the plain, where the Battle of Thermopylae was probably fought, would have been less than wide. Between 480 BC and the 21st century, the shoreline advanced by as much as in places, eliminating the narrowest points