While it's true there were 300 Spartians at the Battle of Thermopolyae, what Frank Miller completely neglected was the 5000 greeks that made the bulk of the opposing forces against the Persians. Upon discovering the overwhelming size of the Persian force, King Leonidas dismissed the majority of the forces saving for 700 Thespian volunteers, and 300 hand-picked Spartians. (Forgot someone, Mr. Miller?)
The 1000 soldiers served as a front line, serving as a road block for Persia allowing the majority of the Greek forces to retreat. By maintaining this front line, Leonidas created the illusion that the full greek force was present. Quite a bit more interesting than the movie.
This bronze helmet was from that battle. British Museam, London, UK.