How do the weak win wars? The likelihood of victory and defeat in asymmetric conflicts depends on the interaction of the strategies weak and strong actors use. Using statistical and in-depth historical analyses of conflicts spanning two hundred years, Ivan Arreguin-Toft shows that, independent of regime type and weapons technology, the interaction of similar strategic approaches favors strong actors, while opposite strategic approaches favors the weak. This new approach to understanding asymmetric conflicts allows us to makes sense of how the United States was able to win its war in Afghanistan (2002) in a few months, while the Soviet Union lost after a decade of brutal war (1979-1989). Arreguin-Toft's strategic interaction theory has implications not only for international relations theory, but for policy makers grappling with interstate and civil wars, as well as terrorism.