Eldest son of Hystaspes, king of the Persians; subdued subject places that had revolted, reorganised the empire, carried his conquests as far as India, subdued Thrace and Macedonia, declared war against the Athenians; in 492 B.C. sent an expedition against Greece, which was wrecked in a storm off Athos; sent a second, which succeeded in crossing over, but was defeated in a famous battle at Marathon, 490 B.C.