as someone who loves history, not much is actually known about the battle at Thermopylae and King Leonidas and his 300 men, the movie got that part right, but everything else was and is left up for interpretation, meaning what we do know came from the Persians
Herodotus a Greek historian is what help validate the claims that took place on that day, the Greek world took Leonidas death hard and viewed them as a marter for the cause: 300 Rise of an Empire depicts this very well and i would say watch them in sequence of each other, meaning to gain the larger picture of what it would have been like and the reasons for why they chose to fight.