Once the battle is joined, the Persian troops prove to be lightly armored, and the Greeks inflict massive casualties. Despite being underequipped, the Persian forces fight bravely and push the Greeks backwards for a short time. But the superior Spartan discipline shows through, and their enemy are routed after intense fighting. Many Spartans are injured, and Dienekes loses his left eye.

The battle continues that first day with both the Persians and Greeks rotating the companies fighting on the frontline. Ariston is slain, as is Olympieus. The fourth wave is made up of the Immortals, Xerxes’s household guard comprised of 10,000 men. Before the Immortals advance to the Greek lines, Leonidas tells the men that this will be the easiest fight of the day:

[T]he troops Xerxes throws at us now are, for the first time, of actual Persian blood. Their commanders are the King’s own kinsmen; he has brothers out there, and cousins and uncles and lovers, offices of his own line whose lives are precious to him beyond price […]The nations he has sent against so far have been mere vassal states, spear fodder to such a despot […] These he treasures.