For over 2,500 years, the forbidding territory of Afghanistan has served as a vital crossroads for armies and has witnessed history-shaping clashes between civilizations: Greek, Arab, Mongol, and Tartar, and, in more recent times, British, Russian, and American. When U.S. troops entered Afghanistan in the weeks following September 11, 2001, they overthrew the Afghan Taliban regime and sent the terrorists it harbored on the run. But America's initial...