As darkness fell, the Persian king sent his best soldiers to take the secret path and come up behind the Greeks.  At dawn on the third day of battle, the Greeks discovered that they had been betrayed.  Leonidas, the Spartan leader, chose to fight to the end.  He knew that his men could never win this battle.  So Leonidas told the remaining Greek soldiers to flee while they could, while the Spartans would hold off the Persians until the other Greeks had retreated.