The Greek city-states have trouble with Persia for a while witch built up into a war.
The Persians had 25,000 soldiers and the Athens ha 10,000. The Persians wore light armor which provided to be a big mistake. The Persians were not prepare and lost over 6,000 solders and the Athens lost 192 men.
The Greek city-states were not untied an couldn't agree on weather or not fight. 300 Spartans decided to intervene at a mountain pass to slow the Persian advance. They were greatly outnumbered and lost the courageous battle.