America's war in Afghanistan, which is now in its fifteenth year, presents a mystery: how could so much money, power, and good will have achieved so little? Congress has appropriated almost eight hundred billion dollars for military operations in Afghanistan; a hundred and thirteen billion has gone to reconstruction, more than was spent on the Marshall Plan, in postwar Europe. General David Petraeus, a principal architect of U.S. counterinsurgency strategy, encouraged the practice of pumping money into the economy of Afghanistan, where the per-capita G.D.P. at the time of the invasion was around a hundred and twenty dollars. He believed that money had helped buy peace during his command of American forces in Iraq. "Employ money as a weapons system," Petraeus wrote in 2008. "Money can be 'ammunition.' "