The situation
In 480 BC Xerxes I led one of the largest armies of the ancient world into Greece. The Greek city-states that chose to resist needed time: their fleet was gathering, and many cities had not committed.
The pass at Thermopylae was the answer to that problem. Between the mountains and the sea, the road narrowed so sharply that numbers counted for far less than discipline.
The decision
Leonidas held the pass for two days. On the third, the Persians learned of the Anopaea path over the mountains, traditionally said to have been betrayed by a local man named Ephialtes, and sent troops around the Greek position.
Warned that he would be surrounded, Leonidas dismissed most of the army. He stayed with his Spartans, the Thespians, who refused to leave, and a Theban contingent. Historians still argue over why: a religious obligation, a deliberate sacrifice to cover the retreat, or a soldier's refusal to give ground.
What it cost
The rearguard was destroyed and Leonidas was killed. Persia took the pass and went on to burn Athens. But within weeks the Greek fleet won at Salamis, and the stand at Thermopylae became the defining story of the war.