In the movie "300" a lot of it is based on Frank Miller's comic book about the 480 B.C. period. Thermopylae is said to have been an important turning point in the history of Greece and the birth of democracy. Spartan King Leonidas II of Sparta led his troops in a fight against a Persian force.
300 is the account of how 300 Spartans fought ONE MILLION PERSIANS! Also, it's not entirely historically correct, since some claim it was probably more like a few thousand than a few tens of thousands, but why let fact get in the way of a nice story?
References
Riffs and Reviews Decker Shado Robocop movie review Decker Shado's site