How were the small city-states able to defeat the mighty Persian Empire

Enough of the cities temporarily put aside their usual rivalriesand in-fighting to form a united front against the Persian Empire.They used superior strategy and tactics – first defeating thePersian fleet to eliminate the Persian amphibious threat to theircities, and thus allow them to unite their armies, where thesuperiority of their armoured soldiers over the unarmoured Persianinfantry gained them land victory and turned back the Persianinvasion.