490 B.C. Athens defeats the Persians at Marathon
Darius, the Persian emperor, was bent on expanding his realm into the Greek world. The help given by the newly-democratized city of Athens to Greek cities in Asia Minor in their resistance to Persia persuaded Darius to invade their territory with a huge fleet and army. He landed his army at Marathon, some 25 miles from Athens, because it was an area supposedly loyal to Hippias, a former Athenian tyrant who accompanied the Persians and who hoped to be restored as a puppet ruler under Darius.
Sparta declined to send help immediately to its fellow Greeks, using the excuse of a religious festival, but the small city of Plataea sent 1,000 men to aid the Athenian force of 9,000 hoplites — heavy infantry. Under Miltiades the Athenians blocked the Persians from moving inland and forced a battle, despite being outnumbered by at least 2 to 1. The Persians seem to have been taken by surprise by a sudden Greek charge, panicked, and were slaughtered in great numbers on the beach.
The Athenian victory was important because it convinced Greece that Persia was not unbeatable and because, had they lost the battle, their experiment with democracy would have ended.
Ten years later, the son of Darius, Xerxes, returned to Greece with an even larger force and confronted the Hellenes in battles at Thermopylae, Salamis, and Plataea.