The Battle of Thermopylae was more than a clash of armies. It was a collision of military systems. Among the vast host of Xerxes I marched his most disciplined corps: the Immortals. Ten thousand strong, constantly replenished, drawn from Persian and Median nobility, they embodied imperial permanence. Herodotus named them for their unchanging number. When one fell, another stepped forward. To Greek observers, they seemed deathless. At Thermopylae, they were unleashed after initial Persian assault

Battle of Thermopylae Explained: Battlefield + How Persia Outflanked the Spartan Defense - w/ Maps