They WERE fighting to defend Sparta in particular, because though they had no great liking for the Athenians, the Spartans had no navy, and the Persian navy was like 4 times the size of the Athenian one. So basically both Athens and Sparta knew they had no chance of beating the Persians one on one, and they teamed up. The two people everyone should know about involved in this are the Athenian commander Themistocles, and of course Leonidas. (incidentally, Leonidas was not the last one to die, he was killed by arrows about halfway through the final day, and the Persians were under orders to take his body/head as a prize, but the remaining Spartans fought so hard to protect it that the Persians wouldn't go near the last like 50 guys, so they called the archers up and buried the whole lot of them with arrows)
Also, Sparta throwing the messengers into a pit is historically accurate, but it should be said that the Athenians threw the messengers sent to THEM into a well instead of a pit.
On topic, once I get an HDTV I will be buying every movie from then on on Blu Ray.