Coercion--the use of threatened force to induce an adversary to change its behavior--is a critical function of the U.S. military. U.S. forces have recently fought in the Balkans, the Persian Gulf, and the Horn of Africa to compel recalcitrant regimes and warlords to stop repression, abandon weapons programs, permit humanitarian relief, and otherwise modify their actions. Yet despite its overwhelming military might, the United States often fails to...