Why did the Greeks, outnumbered, defeated the Persians?

The Greeks had a smaller army, but it consisted only of the Greeks who fought for their country. The Persians, on the other hand, had mercenaries who did not have the same motivation for victory as the Greeks. Greece had a strong navy, which was important in any war because it added power. The Persians had a place to reassess their capabilities: they did not expect such a strong resistance from the Greeks, so they did not calculate their strength.