Afghanistan today is known mainly for its hounds, carpets and pistachio nuts. Its rugged, ruin-strewn terrain is still strategically important, the geopolitical crossroads between China, Russia, India and Iran. But centuries ago it was a well-traveled highway. Remarked Hsüan-tsang, a 7th century Chinese Bud dhist pilgrim, of this 800-mile bridge between the East and West: "Here are found objects of merchandise from all parts."
Over the centuries the debris of a dozen cultures has piled up alongside its ancient caravan routes. In 1500 B.C., the Aryans swept through to invade India. In the 4th century B.C., Alexander the Great's...