This touches on 300's most noteworthy abuse of history: the Persians are turned into monsters, but the non-Spartan Greeks are simply all too human. According to Herodotus, Leonidas led an army of perhaps 7,000 Greeks. These Greeks took turns rotating to the front of the phalanx stationed at Thermoplyae where, fighting in disciplined hoplite fashion, they held the narrow pass for two days. All told, some 4,000 Greeks perished there. In 300 the fighting is not in the hoplite fashion, and the Spartans do all of it, except for a brief interlude in which Leonidas allows a handful of untrained Greeks to taste the action, and they make a hash of it. When it becomes apparent they are surrounded, this contingent flees. In Herodotus' time there were various accounts of what transpired, but we know 700 hoplites from Thespiae remained, fighting beside the Spartans, they, too, dying to the last man.
No mention is made in 300 of the fact that at the same time a vastly outnumbered fleet led by Athenians was holding off the Persians in the straits adjacent to Thermopylae, or that Athenians would soon save all of Greece by destroying the Persian fleet at Salamis. This would wreck 300's vision, in which Greek ideals are selectively embodied in their only worthy champions, the Spartans.
This moral universe would have appeared as bizarre to ancient Greeks as it does to modern historians. Most Greeks would have traded their homes in Athens for hovels in Sparta about as willingly as I would trade my apartment in Toronto for a condo in Pyongyang.