Over the next five years, the Spartans continuously campaign to force new cities into the alliance. In the meanwhile, Darius, the Persian emperor, dies. Contrary to the hopes of the Greeks, Xerxes, the new emperor, does not disperse the invasion force. Phobos, or fear, spreads throughout Greece, making people desperate and causing smaller Greek cities to submit to Persia. Among the greater city-states, only Sparta, Athens, and Corinth remain committed to the alliance.

Dienekes is dispatched on numerous embassies to try and convince the smaller cities to reconsider, but he is unsuccessful. The Persian invasion force crosses into Europe. A counterforce is dispatched to meet them but returns when the chosen spot proves indefensible.

After the allied Greeks have dithered for a while, word comes that Thermopylae has been chosen as the field of battle. It is soon learned that only 300 Spartan Peers—full citizens with male offspring—will be sent to hold the pass at Thermopylae. It is immediately understood that this is a suicide mission. Dienekes, who has no sons, is not eligible to join the Peers.