With the fact that this happened before America was even a sparkle in England's eye SHOULD be proof enough that this movie does not have propaganda
Quite simply, you're wrong.
The MOVIE was made when America was the dominant world superpower. It is impossible not to put a modern lens over anything created in the modern day. They have to, to make it make sense to today's public. Follow the history texts- the slants on real events change over time.
300's script involves quite a lot of Americanism- the Spartans spent their whole time fighting for 'Freedom,' and 'Reason' over 'Eastern Mysticism.' (direct quotes there)
No Spartan ever had a battle cry that included the word 'reason.' They were crazy fuckers.
And was it ever recorded in history the position in which the king of the Spartans died? Specifically 'with his legs straight and his arms out to either side, not entirely unreminiciant of the way Jesus died.' Somehow I doubt it. The whole 'BC' gives it away a little bit.
I really enjoyed the movie, btw. Fantastic graphics, and choreography, and nice, bloody plot. But it is massively American loving.