A second expedition, commanded by Artaphernes and Datis, destroyed (490) Eretria and then proceeded against Athens. The Persians encamped 20 mi (32 km) from the city, on the coast plain of Marathon. Here they were attacked and decisively defeated (Sept.) by the Athenian army of 10,000 men aided by 1,000 men from Plataea. The Athenians were heavily outnumbered, but fought under Miltiades, whose strategy won the battle. They had sought the help of Sparta, by way of the Athenian courier Pheidippides, who covered the distance (c.150 mi; 241 km) from Athens to Sparta within two days. The Spartan forces, however, failed to reach Marathon until the day after the battle.

The Persians did not continue the war, but Darius at once began preparations for a third expedition so powerful that the overwhelming of Greece would be certain. He died (486) before his preparations were completed, but they were continued by Xerxes I, his son and successor. The Athenians were persuaded by their leader Themistocles to strengthen their navy. In 480, Xerxes reached Greece with a tremendous army and navy, and considerable support among the Greeks. The route of the Persian land forces lay through the narrow pass of Thermopylae. The pass was defended by the Spartan Leonidas; his small army held back the Persians but was eventually trapped by a Persian detachment; the Spartan contingent chose to die fighting in the pass rather than flee. The Athenians put their trust in their navy and made little effort to defend their city, which was taken (480) by the Persians.

Shortly afterward the Persian fleet was crushed in the straits off the island of Salamis by a Greek force. The Greek victory was aided by the strategy of Themistocles. Xerxes returned to Persia but left a military force in Greece under his general, Mardonius. The defeat of this army in 479 at Plataea near Thebes (now Thívai) by a Greek army under the Spartan Pausanias (with Aristides commanding the Athenians) and a Greek naval victory at Mycale on the coast of Asia Minor ended all danger from Persian invasions of Europe. During the remaining period of the Persian Wars the Greeks in the Aegean islands and Asia Minor, under Athenian leadership (see Delian League) strengthened their position without seeking conquest.