The human skeletons were piled up like signposts in the sand. For Xuanzang, a Buddhist monk traveling the Silk Road in A.D. 629, the bleached-out bones were reminders of the dangers that stalked the world's most vital thoroughfare for commerce, conquest, and ideas. Swirling sandstorms in the desert beyond the western edge of the Chinese Empire had left the monk disoriented and on the verge of collapse. Rising heat played tricks on his eyes, torturing him with visions of menacing armies on distant dunes. More terrifying still were the sword-wielding bandits who preyed on caravans and their cargo—silk, tea, and ceramics heading west to the courts of Persia and the Mediterranean, and gold, gems, and horses moving east to the Tang dynasty capital of Changan, among the largest cities in the world.