Cyrus II, chief of the Persian tribe in the southwestern Iranian plateau, overthrew his maternal grandfather Astyages and the Median state in 550 BCE. Within a single generation Lydia (taking Sardis in 547) and Babylon (539) had fallen to him; his son Cambyses II added Egypt (525), and Darius I (522–486) extended the empire east to the Indus valley and west into Thrace. At its height the empire covered about 5.5 million km² and an estimated 50 million people — perhaps forty percent of the world's population at the time.

The lasting Achaemenid innovation was administrative. Darius divided the empire into about twenty *satrapies*, each remitting a fixed tribute and inspected by royal officials known as "the king's eyes". The Royal Road from Sardis to Susa, some 2,700 km long, could be crossed by mounted couriers in a week. Local languages, religions and laws were largely preserved — Cyrus's permission for the Jewish exiles to return to Jerusalem after the conquest of Babylon (recorded in Ezra/Nehemiah and echoed on the Cyrus Cylinder) is the best-known expression of this policy of tolerance.

The empire fought two famous wars with the Greek city-states (Marathon in 490, Salamis in 480). These episodes loom so large in Western historiography that the Persian side is often remembered as "the loser" — yet the Achaemenids reigned for another century and a half. Their end came with Alexander's conquest (334–330 BCE), but their administrative legacy — the satrapy system, the multi-ethnic state, the *Pax Persica* — passed to the Hellenistic kingdoms, then to Rome and Byzantium. Persia was the first sustained demonstration that the world could be governed.