History.
Now, i don't want to make a 1000 pages thread about all the things that are historically wrong in this movie.
It's a fine action flick after all, but many people seem to take this movie as a historical movie. This it is not.
I urge everyone who wants to know what actually happened to watch this 12 minute long video
https://www.youtube.com/watch?v=Q-mkVSasZIM
If you want to know more, just go to wikipedia and read some of the articles about Sparta, the Persians, the wars they fought and so on.
I think it is very important to tell the difference between fiction based on history, and actual history.