The correct answer is Thermopylae.
In 480 BC, King Leonidas of the city-state of Sparta was at war with the Persians. During the battle, he held back a vast army of the Persians at the narrow costal pass of Thermopylae. But he was betrayed by a local resident called Ephialtes who revealed to the Persians that a small path led behind the Greek lines.
Realizing that he had been outmaneuvered by the Persians, Leonidas sent away the bulk of his Army and with 300 of his fellow Spartans that opted to stay with and fight to death, remained at the pass to guard his army's retreat. Even though the Greeks lost this particular battle, Leonidas death was seen as a heroic sacrifice, which gave his army that he had sent away a chance to live and fight another day.
So the Greeks made a tactical withdrawal and a little later struck back at the Persian army and decisively defeated them at the battle of salamis and battle of Plataea, thereby ending the Persian invasion.
