The story as presented in the film in purely fantastical embellishment created for propaganda and morale purposes, to beef up the Spartans for their later defeat of the Persians after Thermopyle. The objective of the story in the comic and film is to show the Battle of Thermopyle after the Spartan equivalent of Goebbels gets his hands on it and beefs it up with war-rhinos and ten-feet-tall Persian god-kings.

Admittedly this is not transmitted very clearly in the film (which should have suddenly cut at the end to a much more realistic-looking Greek army of heavily-armoured soldiers rather than warriors still protected by their battle-shorts). Zack Snyder is not good at doing 'subtle'.

As a retelling of the real story of the battle, it is entirely worthless. In fact, the film may have significantly damaged the chances of us seeing Steven Pressfield's historically accurate GATES OF FIRE ever being transferred to the screen, at least for a few more years until memories of 300 die away. However, 300 is so insanely over the top and divorced from reality, I thought it was pretty clear that this was not supposed to be a historically accurate retelling of the story. It's about one step away from expecting historical accuracy in the ASTERIX comic books.