The Battle of Thermopylae - Persian View

Sparta... free... right... 35,000 of the hellenic troops at Plataea were helot slaves.

Actually there were 7,000 Greeks and the Persians likely had around 200,000 given the nature of the Persian empire this number is not unlikely.  Especially since many of these troops are poorly equipped and with almost no training.  Consider the accounts of Cstesias who described portions of the Persian force as being armed with no more than daggers.  As for the Greeks we know that they were on average far more heavily armed and typically much better trained.  Furthermore the numerical disadvantage was nullified by two very important factors.  First the narrowness of the pass which negated the numberical advantage of the Persian army.  Second the numerical disparity was not as great as it seems on account that Leonidas had set up a relay system with nearby cities in order to rotate his troops and maintain fresh troops at his front.  Its possible that while the Greek army present might have never exceeded 7k that perhaps as many as 12 might have fought at thermo over the course of the battle.  Now asides from the factors just mentioned I would argue that the vast majority of Persians killed at Thermopylae found their end not at the tips and blades of the spears and swords of Greece but by the fearsome feet of their compatriots.