The reason why Leonidas appeared with only a token force was that Sparta was at that time celebrating a religious festival; the reason why the other Greek forces were so scanty was that their cities were celebrating their Olympic games. Neither Sparta, famous for the quality of her fighting men, nor her allies thought that the battle of Thermopylae would be engaged as soon as it was, so the matter stood and there was nothing to be done about it: The Greek forces were hopelessly outnumbered, the Persian forces were upon them and in command of the pass through which they were penetrating the country, and there was Leonidas with his 300 Spartans camped in front of the enemy's first wave.
Xerxes, the tyrannical ruler of the Persians and their huge conglomerate of allies, was — like many tyrants of his time and later — an unstable and arrogant person. Earlier in the war, having enthroned himself upon a vantage point overlooking his entire war host, he had alternately laughed at the earthly military might he saw displayed before him and then wept at their mortal mutability and evanescence. At any rate, Xerxes the tyrant was resolved to tolerate no insolence from the effete and intellectual Greeks who called him a barbarian, and the appearance of a mere 300 Spartans to engage his host in hand-to-hand combat must have seemed insolent in the extreme.
Xerxes was at the same time angered and intrigued by these men called Spartans, so he had the Greek ramparts scouted out. On the day that Xerxes sent his scout to reconnoiter the Greek camp, the Spartans had been assigned as perimeter guards outside the camp's ramparts. There, Xerxes' scout saw them, counted them, and then returned to report to his master what he had seen.
