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Stunned by the terrorist attacks of September 11, 2001, the Bush administration adopted a new National Security Strategy in September 2002. The UK government took a similar stance. This new strategy calls for anticipatory attacks against potential enemies with uncertain capacities and intentions, even before their threat is imminent. Rather than wait for evidence of weapons of mass destruction, it shifts the burden of proof, obliging ‘‘rogue’’ states to show that they do not harbor weapons of mass destruction or terrorist cells, or else face the possibility of attack. This new strategy amounts to the adoption of the Precautionary Principle against the risk of terrorism. We offer two main conclusions about precaution against terrorism. First, any action taken to reduce a target risk always poses the introduction of countervailing risks. Moreover, a precautionary approach to terrorism is likely to entail larger, more expensive interventions, so the expected opportunity costs are likely to be higher. While considering worst-case scenarios is important for the development of sound policy, taking action based only on worst-case thinking can introduce unforeseen dangers and costs. We argue that a better approach to managing risk involves an assessment of the full portfolio of risks—those reduced by the proposed intervention, as well as those increased. We argue that decision makers developing counterterrorism measures need mechanisms to ensure that sensible risk analysis precedes precautionary actions. Such a mechanism currently exists to review and improve or reject proposed precautionary measures against health and environmental risks, but not, so far, for counterterrorism and national security policies. We urge the creation of such a review mechanism.