The 300 Spartans

The original 300.
And probably more closely related to the actual Battle of Thermopylae than its modern rival.
The 300 Spartans is based on the the Battle of Termopylae, which saw one of the greatest last stands in military history. In 480BC, King Leonidas of Sparta led a rearguard action with 300 Spartans, 700 Thespians and 400 Thebans as they held out an invading Persian army ranging in number from 100,000 to 150,000 (depending on where you get your information). They managed to repel the invading force by plocking a small coastal road, and were only defeated when Ephialtes, a local resident, betrayed the Greeks by revealing a path that led behind their defensive line.
The 1962 film attempts to depict this infamous battle, with Richard Egan taking on the lead role of King Leonidas.
For the action-savvy viewer, you have to cool your heels for quite some time as the film dedicates quite a bit of time to the build-up. This includes a few diplomatic meetings, and Xerxes of Persia (David Farrar) announcing his intention to defeat the city-states of Greece.
Keeping in mind it was made/released in 1962, the film isn't too bad. Being the fan of action that I am it took a while for me to get into it, but it is very much worth a viewing. An old school classic.

The 300 Spartans: 109 minutes (1962)