[Japo]

The Persian army was fearsome in the East and its numbers were huge, but it couldn't be compared with the Greek hoplites either in terms of tactics, equipment or morale. Even the Immortals' equipment was very inferior to a run-of-the-mill citizen hoplite's. Also artillery wasn't effective against troops until Modern times, and the bombs in the film are of course grossly unhistorical. It was because the Greek troops couldn't stand a fight in the open due to their vastly inferior numbers, that they were lost once they were surrounded. The reason why Thermopylae was chosen was because the Persians' numbers were to no avail in so confined a battleground. The only thing the Persians could do about that was shooting arrows, which actually was one of their primary tactics --as most Persian footmen were worthless in melee but were devastating when shooting simultaneously--, but precisely the Greek hoplites' equipment made them as impervious to arrows as possible.

Also if as you say Persian troops were qualitatively comparable to Greek ones or better, the Athenians couldn't possibly have sacked Sardis nor fended off the Persian retaliation in Marathon; in the latter case the battle was out in the open, and the Athenians were vastly outnumbered although not of course as much as in Thermopylae.

I stand by my opinion that although we can't trust ancient (or present) historians to every word, we must understand that they were generally trying to record facts in good faith. As I said most times we've been able to check the faithfulness of ancient sources, they've proven themselves surprisingly accurate.