What was the secret ingredient that made the Greeks so formidable a foe that the greatest empire in the world failed to come even close to defeating them?

Tag words: Herodotus; The Histories; Battle of Plataea; Greeks; Pausanias; Xerxes; Mardonius; Sparta; Leonidas; Thermopylae; Vassily Grossman; A Writer at War; Great Patriotic War; Krasnaya Zvezda; Red Star; John Gould; Richard Cohen; Making History; Victor Davis Hanson; Carnage and Culture; Persians; Marathon; Battle of Salamis; Artemisium; Cyrus the Younger;