[DwArD]

Hmm....I thought the reason they didn't went back and brought in reinforcements was the reason why they only left with 300 spartans right at the start of the story? Its not that Leonidas don't wanna bring more troops...its just that he can't. Every matter that concern Greek must first went through oracle of some sort, and in the case of the movie, the oracle (plus the old perverted geeks) forbide Leonidas from going into battle with the Persians. But Leonidas didn't give up and decided to bring along with him 300 of the finest Spartans. Hope that solves your doubt. Btw, you can take a look at wikipedia's article on 300 and the Battle of Thermopylae. Its pretty enriching.

Btw, the only thing that puzzled me was why didn't Leonidas aimed the spear right at Xerxes's heart? I know its very difficult to aim it from that distance but from what I read from wiki, the comic mentioned that Leonidas's goal was to show the people that Xerxes isn't a "God" and that he bleeds.