The area that is present-day Afghanistan comprised several satrapies (provinces) of the Achaemenid Empire when it was at its most extensive, under Darius the Great (ca. 500 B.C.). Bactriana, with its capital at Bactria (which later became Balkh), was reputedly the home of Zoroaster, who founded the religion that bears his name.

By the fourth century B.C., Iranian control of outlying areas and the internal cohesion of the empire had become tenuous. Although outlying areas like Bactriana had always been restless under Achaemenid rule, Bactrian troops nevertheless fought on the Iranian side in the decisive Battle of Gaugamela (330 B.C.). They were defeated by Alexander the Great.

Source: U.S. Library of Congress