The Spartans United with the Greece to fight against the Persian army lead by Xerxes. 300 spartans went in front and protected the small pass killing about 20.000 of the 350.000 persian warriors. The rest of the Spartan army was waiting at Sparta to defend the City... So no, They didn't fight against the Greece. Greece Just offered to help. You can also see that in the movie when the spartans are on their way to the pass, they meat a Larger army of Greece people, the spartans laugh because they are no Professional Soldier.
Anyway, Trying out the map now. ;D