When do states attack or consider attacking nuclear infrastructure in nonnuclear weapons states? Despite the importance of this question, relatively little scholarly research has considered when and why countries target nuclear programs. The authors argue that states are likely to attack or consider attacking nuclear facilities when they are highly threatened by a particular country’s acquisition of nuclear weapons. Three factors increase the salience of the proliferation threat: (1) prior violent militarized conflict; (2) the presence of a highly autocratic proliferator; and (3) divergent foreign policy interests. The authors test these propositions using statistical analysis and a new data set on all instances when countries have struck or seriously considered striking other states’ nuclear infrastructure between 1941 and 2000. The findings lend support for the theory and very little support for the alternative explanations. States are not deterred from attacking nuclear programs by the prospect of a military retaliation and concerns about international condemnation do not appear to influence the willingness to strike. Ultimately, states are willing to accept substantial costs in attacking if they believe that a particular country’s acquisition of nuclear weapons poses a significant threat to their security.