We had an interesting post from Leonidas himself back in April - https://amp.reddit.com/r/AskHistorians/comments/mhrjdw/review_this_movie_300_is_a_vicious_slander/

There was a guy named Leonidas who led Spartans to a place called Thermopylae where there was a fight against the Persians under the Great King, Xsayarsa. Everything else is...iffy, at best.

More can always be said, of course; for the meantime, here's u/Iphikrates going over the film itself, plus Episode 116 of the Podcast covers the battle and the film.