CYRUS II (c. 585–c. 529 BCE), called Cyrus the Great; builder and ruler of the Persian empire from 559 BCE until his death. A king of the Achaemenid dynasty, Cyrus (OPers., Kurush) combined great ambition, shrewd calculation, and military expertise to establish the largest empire in world history. From his base in Anshan he conquered neighboring Media in alliance with the Babylonian king Nabonidus in 550, overtook Lydia in Asia Minor in 547, defeated resisting areas in the Greek mainland, then returned to Persia and drove his armies eastward as far as India. With his power thus increased, he conquered Babylonia and proclaimed himself king of all Mesopotamia—indeed, of the world—in 539.

Nabonidus had alienated the Babylonian priesthood through his extraordinary devotion to the moon cult. Capitalizing on Nabonidus's heresy, Cyrus achieved popularity in Babylon by restoring the cult of its chief god, Marduk, and by reestablishing the shrines...