Darius dies in 486 bc and is succeeded by his son xerxes who invades greece in 480: during this time, athenians built a new fleet. Persian army while on the way down to attica went by land, aided by their naval support. Greeks needed time to organize their forces so they made a stand at thermopylae. Thermopylae hot gates : a narrow pass with a mountain cliff on one side and the sea to the other. Around 6700 troops (hoplites from other city-states) and 300 spartans by king leonidas made their stand: around 1500 greeks died while 20,000 persians were killed, including two of. Greek named ephialtes defected to the persians and showed them a passage around. Thermopylae so they could surround the spartans from both sides: ephialtes became synonymous with traitor . Lenoidas was told by the oracle at delphi that either sparta would be destroyed or it would lose its king explanation as to why he stayed.