Balkh (bälkh) [key], town, N Afghanistan, on a dried-up tributary of the Amu Darya River. One of the world's oldest cities, it is the legendary birthplace of the prophet Zoroaster. Because it was located on natural travel routes at a source of water, the town was important as early as the 3d millennium B.C., when the lapis lazuli trade to Mesopotamia began. Alexander the Great reputedly founded a Greek colony at the site c.328 B.C. The city later attained great wealth and importance as Bactra, capital of the independent kingdom of Bactria. In the early centuries A.D., Balkh, a prominent center of Buddhism, was renowned for its Buddhist monasteries and stupas. Conquered by the Arabs in the 8th cent., it became important in the world of Islam as the original home of the Barmakids. Under the Abbasid caliphate its fame as a center of learning earned Balkh the title "mother of cities," and the Persian poet and mystic Jalal ad-Din Rumi was born there. The city was sacked in 1221 by Jenghiz Khan and lay in ruins until Timur rebuilt it (early 16th cent.). It passed to the Uzbeks and then briefly to the Mughal empire before falling (18th cent.) to Nadir Shah. In 1850, Balkh became part of the unified kingdom of Afghanistan. The old city is now mostly in ruins; the new city, some distance away, is an agricultural and commercial center, inhabited chiefly by Uzbeks.