Plus this is historical fact. In 481-480 B.C., King Xerxes of Persia raised forces in Asia and Africa and invaded Greece with an army so huge it was said it (the army) "drank rivers dry." But when they entered the mountain pass of Thermopylae they encountered 300 soldiers from Sparta (King Leonidas and his 300-man bodyguard). Hence the title "300".
Basically the battle bought the Greeks enough time to escape and defeat the Persians. Later, much later.
There were more than just Spartans, unfortunately they are not given their due, except by the Spartans and some historians. The Thespians held out their part of the bargain very well.
There were more than 300 total peeps though the number of spartans was indeed 300.
The significance of the battle was two-fold. Primarily it was a huge slap in the face of Xerxes. More importantly though, it galvanised the fractured greek city states to take up arms as one and they fought with a resolve that hitherto had not existed, allowing them, eventually, to repulse the Persians.
Alexander would finish the job much, much later.
This movie though looks bloody good, now that I've seen more of it in previews and so forth
Can't wait for it to be released and I don't give a crap if there is a lot of cgi in the background if it is used like Sin City to further the story rather than to drown it in crap.