Termopylae After the Athenians beat the Persians in the First Persian War, at the battle of Marathon, the Persians left the Greeks alone for ten years. The Persians were busy fighting a revolt in Egypt, and their king Darius had died. But as soon as Darius' son Xerxes (ZERK-sees) settled the Egyptian revolt, he began to plan how he would conquer those terrorists in Greece.