The defeat at Thermopylae left Athens vulnerable so the city had to evacuate.
Most people escaped to salamis, which was a large island off the coast of Athens.
Themistocles put all able bodied men on ships and assembled the Greek fleet in the straits of Salamis. He was sure their best chance to defeat the Persians lay in fighting them in those narrow waters which the Athenians knew better than the Persians.
Themistocles sent a messenger to Xerxes with flase information saying that the Greeks were about to sail away.
Our army will sail away today.
Xerxes ordered the Persian army to attack the following day.
Eager for victory, Xerxes moved his fleet to the narrow gulf.