Okay, Nomad is right about Leonidas being the right man to save Greece.... but here are some things the movie Leonidas did that will bring a would be emulator to disaster:
* Leonidas treats friends the way most people treat enemies.
Yeah, we get it already: You are the few, the proud, the Spartans. But let's emphasize a key point - you are the few. You'll surely need the other 97% of us to win this war - and who wants to watch your flank with that attitude of yours? It's all nice to put down the Arcadian troops as being 'only' blacksmiths and farmers - but exactly who grows all the food, makes the cloaks and weapons so you have the luxury of practicing war all day long?? The fact that these guys are going to face the same enemy as you without all the training makes them as brave or braver in my book!
Leonidas fails where many of us who have achieved something fail: Our pride in our accomplishments too easily turns into derision of others. In coalition war, it will cripple your effort.
* Leonidas Murders Everybody - Except the Guy He Should Have Killed
What's with throwing messengers down into the pit? Looks fun, feels good at the moment - but some day you may answer for it. Also, need one be a great warrior to butcher all the wounded guys? In real life, that'll mean an enemy army unit that would have quit the field will instead fight to the death - resulting in needless deaths of your own troops as well.
Leonidas stops his captain from killing the one guy they arguably should have off'd - the hunchback. The guy knows the terrain, but you have to reject him and send him away angry. Hey, you're killing everyone else in the cast, why not toss this suspect off the cliff - - - and the Persians don't find the path to your rear.
Or, better yet, do what another great swordsman - Musashi - would have done and find some use for the guy. (Note the carpenter section in his Book of Five Rings)