The expansion of U.S. counterterrorist operations throughout the world coincides with a growing sense among some U.S. policymakers that so-called "failed states" pose grave threats to international security. The governments of failed states have weakened to the point that they can no longer provide public goods, such as territorial integrity, economic infrastructure, and physical security. U.S. defense strategists suspect that the lawlessness of failed states may do more to undermine security in the United States than direct confrontation by hostile governments. Denying terrorists the sanctuary they seek in failed states may become a central feature in the war on terror, and it is likely that the United States will use preemptive force against suspected terrorists inside a state that is incapable of policing itself. This Note examines the legality of using preemptive military force against suspected terrorists located in failed states.