Persia had crushed the Greek revolt along the Anatolian coast between 499 and 493 (the Ionian Revolt); the support that Athens and Eretria gave the rebels turned Darius's attention to mainland Greece. In the summer of 490 BCE a Persian force was landed at the plain of Marathon, some 40 km northeast of Athens. The Athenians, commanded by Miltiades and joined by a small Plataean contingent, marched out to meet them. Sparta sent help but, held back by a religious festival, would arrive late.

The engagement was decided when the hoplite phalanx, in heavy armor on level ground, pushed the Persian infantry back. Miltiades thinned his center and reinforced the wings; he let the Persian center advance and then enveloped it from the flanks. Greek losses were on the order of 200; Persian losses are reported in the thousands (precise figures are debated). The Persian fleet attempted a strike at Athens itself, but by the time it reached the harbor the army's runners had already brought word of victory to the city, and the attempt was abandoned.

Marathon did more than halt a single campaign. It set the stage for Xerxes's much larger invasion ten years later (480–479). The confidence Athens drew from the victory fed the consolidation of its democratic institutions and the cultural surge that followed (the age of Aeschylus, Sophocles, and Pericles). The name 'marathon' for long-distance running comes from a later tradition that the messenger Pheidippides ran from the battlefield to Athens with the news — a story not directly attested in the earliest sources, but the source of the modern race's name.