Province of the ancient Persian empire. It was centered in the fertile valley of the Zeravshan River, in modern Uzbekistan. It became a satrapy under Darius I in the 6th cent. BC, and was conquered by Alexander the Great in the 4th cent. It asserted its independence from the Seleucid empire c.250 BC as part of the Bactrian kingdom, and fell to invading N tribes in the 2nd cent. BC. It prospered until the Mongol invasions. Under the Samanid dynasty (9th-10th cent. AD), it was a focal point of Islamic civilization. See also Bukhara.