- Reign of Cyrus II

On the death of his father, Cyrus II became the king of the Persians. In 553, Cyrus led a revolt against his grandfather Astyages. Although he suffered some early defeats, the Median army eventually went over to Cyrus, and he took Ecbatana in 549. Cyrus now ruled the entire Median Empire. In 546 Cyrus conquered Lydia, adding much of Asia Minor to his realm. Cyrus then defeated King Nabonidus, entered Babylon in 539, and took over all the Babylonian Empire: Mesopotamia, Phoenicia, and Syria-Palestine.

- Reign of Darius the Great

A member the Achaemenid family, Darius I defeated Gaumata's revolt as well as other revolts in Babylonia and the eastern provinces. Darius's commemoration of his achievements, the Behistun inscription, written in Old Persian, Elamite, and Akkadian, became the key to the modern decipherment of cuneiform. Darius later added the Indus Valley and Libya to his empire, now the largest the world had ever known. He reorganized the administration and divided the empire into 20 satrapies, as well as introducing a standard gold coinage, the daric. At its height, the Persian Empire probably contained around 15 to 16 million inhabitants, with some 4 million in Persia proper. There were royal residences at Susa, Persepolis, Ecbatana, and Babylon, and good roads, with stations for royal messengers, which made possible regular communications within the vast realm. After 513, Darius started expanding into Europe and led an expedition which crossed the Danube. In 499, the Ionian Greeks in Asia Minor revolted but were suppressed after a six-year war. The Athenians had aided the rebels, and to punish them Darius sent the expedition which was defeated at the Battle of Marathon