Xerxes, for whatever reason, permitted the Spartans and their allies a four-day recess from the rigors of battle, but on the fifth day he attacked, ordering his Medes and Cissians to capture the Spartans and bring them as captives to his camp. The Persians suffered horrendous losses in their offensive because the Greek allies fought so courageously in the narrow defile, employing their long battle lances to great effect. Xerxes then sent in his crack troops (his "Immortals") against the Greeks, who proved the "Immortals" to be inaptly named by killing so many of them. Thus the Persians and the Greeks fought for three days in the Hot Gates, and the Greeks grimly refused to concede defeat. But the next day a traitor to the Greeks, a person named Ephialtes, came and whispered in the king's ear. And again Xerxes laughed.
Xerxes laughed because Ephialtes told him of a secret passage through the mountain, which would bring the Persian troops down behind the Spartan ramparts. So that night Xerxes sent his "Immortals" on their way to attack the Greeks from the rear while another wave of his troops would accost them from the front. As the "Immortals" ascended the mountain, they encountered a contingent of Greeks (Phocians) who were stationed in the Persian line of march. The Phocians fled up the mountain; the Persians advanced down the mountain to fall upon Leonidas and his Spartans as the new day dawned.
So as false dawn appeared, the Greeks at Thermopylae held a war council, where some of the allies voted to stay and fight and some voted to flee. It is said that Leonidas himself ordered the allies to vacate the Spartan ranks, but that the Spartans themselves had no intention of ducking an opportunity to fight. But Leonidas did permit the Thespians to stay and fight alongside him because they wanted to, and he made the Thebans stay and fight because they certainly did not want to. And so most of the allies left; the Spartans remained; and the day came on.
