Darius the Great (r. 522–486 BCE) turned Cyrus’s achievement into a state that could last: crushing usurpation, proclaiming his right at Behistun, and then engineering an empire of laws, roads, coinage, and census—satrapies kept honest by royal inspectors, the daric giving Persia a stable measure of value, the Royal Road binding Susa to Sardis in weeks, and canals linking Nile to Red Sea. He drove Persia’s frontier to the Indus, bridged the Bosporus, tested the Scythians, and raised Persepolis as the ceremonial heart of rule, a terrace of stone where tributary nations climbed to offer gifts beneath winged guardians. If Cyrus founded, Darius systematized: he wrote order into geography, made governance a technology, and gave “King of Kings” its administrative meaning—strength guided by arta, the right ordering of the world.