Part I of this essay reviews cases of deterrence failure and assesses the political, psychological, and practical obstacles to the success of deterrence. The evidence suggests that the utility of deterrence is limited to a narrow range of cases. It is appropriate only when leaders are motivated largely by ' 'opportunity" rather than by "need," have the political freedom to exercise restraint, are not misled by grossly distorted assessments of the political-military situation, and are vulnerable to the kind of threats a deterrer can make. Deterrence must also be practiced early on, before an adversary commits itself to a challenge and becomes correspondingly insensitive to warnings that its action is likely to meet with retaliation. Unless these conditions are met, deterrence will at most be ineffective and at worst counterproductive. Part I1 examines deterrence successes and those cases where deterrence might have succeeded had it been attempted. A case-by-case review makes it apparent that there are few unambiguous examples of the success of deterrence. Inevitably, proponents of deterrence resort to counte$actual reasoning to allege that deterrence would have worked had it been tried. A review of these arguments suggests that they too are frequently open to multiple interpretation. Because the evidence of success is ambiguous and because deterrence can be ineffective, uncertain, and risky, it must be supplemented by other strategies of conflict management. Part III looks beyond deterrence to strategies of "reassurance" that might substitute for or complement deterrence and reduce some of its obvious risks. Strategies of reassurance presume ongoing hostility but, unlike deterrence, root the source of that