The yoke of oppression rested uneasily on the conquered Ionian cities, and in 499 B.C. a revolt was fomented. Athens, which was becoming a center of western civilization with political progress toward democracy, aided the revolution by sending armies. Although the revolt was crushed, the incensed King Darius of Persia decided to punish Athens. In 492 B.C. he organized a huge army and navy to attack the mainland of Greece, but his fleet was destroyed in a storm and his land forces suffered expeditionary difficulties. Two years later the Persian armies penetrated Attica where they were decisively defeated by the Athenians at Marathon. Athens assumed the mantle of Greek leadership.