Darius I, the Great was the greatest ruler of the Achaemenid dynasty and the best organizer of the Persian empire.

Darius became king after murdering the previous king's son who had usurped the throne. As king he organized the Persian Empire into administrative units called satrapies and he built great arterial royal roads to hold the empire together. He undertook major building programs in Persepolis, Susa, Egypt, and elsewhere.

Darius conquered the Indus valley and campaigned against tribes on the frontiers. He suppressed the revolt of the Greek city-states in Asia Minor, but his punitive expedition against Greece failed at the battle of Marathon.