The Achaemenid Persian Empire at its greatest extent controlled territory from the Indus Valley to the Aegean coast — the largest empire the ancient world had yet seen, governed by a relatively small Persian ruling class over dozens of distinct peoples and languages.

Cyrus the Great's approach to conquest was deliberately different from Assyrian policy: rather than deportation and destruction, he restored sanctuaries, repatriated exiled peoples, and allowed conquered peoples to maintain their religious and cultural practices. The Cyrus Cylinder, which describes his capture of Babylon in 539 BC, is one of the earliest royal inscriptions to articulate something like a policy of religious tolerance.

Darius I's administrative reforms — standardized coinage, the satrapal system with dual oversight, the Royal Road with royal postal service — transformed this conquest state into something more durable. The Achaemenid model of governance influenced both Alexander's successors and later empires in the region for centuries.