Who was the spy in Xerxes court? And how genius was the way he got his message to the Greeks that revealed Xerxes’ plans for the invasion of Greece?

Tag words: Xerxes; Spartans; Thermopylae Pass; Persians;  kill ratio; Darius; Herodotus; Histories; Demaratus; best fighting men in the world; Victor Davis Hanson; Carnage and Culture; JFC Fuller; The Decisive Battles of the Western World; John Lewis Gaddis; On Grand Strategy; Salamis;