There is a path around Thermopylae that will allow you to ambush the Greeks.
May you live forever, Ephialtes
The rest of the army must retreat. Some will stay behind to face Persian forces. May Ephialtes live forever!
The troops at Thermopylae have been defeated! Retreat!
213 -The king (Xerxes) being at a loss how to deal with the present difficulty, Epialtes son of Eurydemus, a Malian, came to speak with him, thinking so to receive a great reward from Xerxes, and told him of (p531) the path leading over the mountain to Thermopylae; whereby he was the undoing of the Greeks who had been left there.
The Persians were able to destroy a sizeable portion of the Greek army. Leonidas was forced to call a war council and order a retreat.220 - It is said indeed that Leonidas himself sent them away, desiring in his care for them to save their lives, but deeming it unseemly for himself and the Spartans to desert that post which they had first come to defend.
News of their defeat reached the Greek troops at Artemisium and they decided to retreat as well. This victory allowed Xerxes and his army to travel into southern Greece, which expanded the Persian army even more.