At Thermopylae, the allied Greek nations deployed a small force of between four and seven thousand Greek heavy infantry against the invading Persian army of two million. Leading the Greeks was a force of three hundred Spartans, chosen because they were all "sires" — men who had to have sons who could preserve their blood line, should they fall in battle.
Thermopylae was the only way into Greece for the Persian army, and presented the perfect choke point — a narrow pass bordered by a sheer mountain wall on one side and a cliff drop-off to the sea on the other. This location decreased the advantage of the Persians' numerical superiority. Delaying the Persian advance here would give the Greek allies enough time to ready a larger, main force to defend against the Persians. The battle takes place simultaneously with the sea battle at Artemisium, where the Allied Greek forces hoped to protect the flank of the army at Thermopylae whilst not being cut off themselves. The Greeks were at a disadvantage at Artemisium, as at Thermopylae - the Persians outnumbered the Allies, and most of the Athenian ships were newly built and manned by inexperienced crews - and both sides suffered heavy losses in the sea battle.
Though Xeones is critically wounded in the battle, the Persian King Xerxes orders his surgeons to make every effort to keep the captive squire alive. The book is Xeones' narration of the battle and events leading up to it to Xerxes and his royal scribe as the Persian army advances toward Athens. Much of the narrative explores Spartan society, particularly the agoge, which is the military training program which all Spartan boys must complete to become citizens or Peers. The novel also details the heroics of several dozen Spartans, including their king, Leonidas, the Olympic champion Polynikes, a young Spartan warrior named Alexandros, and the Spartan officer Dienekes. Pressfield employs detailed descriptions of the Spartan phalanx in battle, as well as the superior training and discipline of the Spartan warriors.