The Achaemenid Persian Empire — founded by Cyrus the Great around 550 BCE and destroyed by Alexander the Great in 330 BCE — was the largest empire the ancient world had yet seen, stretching from the Aegean to the Indus, governing perhaps 40 percent of the world's population through a system of administrative tolerance, royal roads, and satrapal governance that became the template for empire across the ancient world.
When Cyrus the Great conquered Babylon in 539 BCE, he did something no conqueror before him had done on this scale: he let the conquered keep their gods. He returned the cult statues that Babylon's previous rulers had seized from subject peoples. He allowed deported populations to go home. The Hebrew exiles in Babylon — whom he freed — called him the Lord's anointed. This was not just mercy. It was a governing system: the Achaemenid Persian Empire ruled fifty different peoples by letting them be who they were, as long as they paid their taxes and provided their soldiers. That system made Persia the largest empire the ancient world had yet produced — and established a model that every subsequent empire has had to reckon with.