We examine the effect of nuclear weapons on interstate conflict. Using more appropriate methodologies than have previously been used, we find that dyads in which both states possess nuclear weapons are not significantly less likely to fight wars, nor are they significantly more or less belligerent at low levels of conflict. This stands in contrast to previous work, which suggests nuclear dyads are some 2.7 million times less likely to fight wars. We additionally find that dyads in which one state possesses nuclear weapons are more prone to low-level conflict (but not more prone to war). This appears to be because nuclear-armed states expand their interests after nuclear acquisition rather than because nuclear weapons provide a shield behind which states can aggress against more powerful conventional-armed states. This calls into question conventional wisdom on the impact of nuclear weapons and has policy implications for the impact of nuclear proliferation.

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The “democratic peace”—the regularity that democracies rarely (if ever) fight with other democracies but do fight with nondemocracies—is one of the most famous findings in international relations scholarship. There is little agreement, however, about the mechanism that underpins the democratic peace. Recently, scholars have shown that mass publics in liberal democracies are less supportive of using military force against other democracies. This finding has been taken to support the idea that the content of public opinion may provide one mechanism that underpins the democratic peace. Using a large-scale survey experiment, we show that mass publics in an authoritarian regime—China—show the same reluctance to use force against democracies as is found in western democracies. Our findings expand the empirical scope of the claim that mass publics are reluctant to use force against democracies, but force us to rethink how public opinion operates as a causal mechanism underpinning the democratic peace.

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What happens to the foreign policies of states when they acquire nuclear weapons? This question has grown in importance as new nuclear powers have emerged and other states have moved closer to joining the nuclear club. Indeed, determining the costs that the United States and others should be prepared to pay to prevent nuclear proliferation hinges on assessing how nuclear weapons affect the behavior of the states that acquire them and how dangerous those effects are. If states expand their interests in world politics or act more aggressively in the aftermath of nuclear acquisition, preventing nuclear proliferation should be a higher priority than if nuclear weapons do not signiªcantly affect the foreign policies of the states that acquire them. Crafting deterrence strategies for new nuclear states also requires understanding the foreign policy effects that nuclear weapons are likely to have in a given case. Despite its importance, the question of how nuclear weapons affect the foreign policies of the states that acquire them has not been satisfactorily answered. The literature on nuclear weapons has generally examined the effects of nuclear weapons on outcomes other than foreign policy; has focused on the effects of nuclear weapons on the calculations of other states rather than the acquiring state; and has often sought to explore how states with nuclear weapons should behave rather than how they do behave. The literature that has examined the effects of nuclear weapons on foreign policy has tended to conºate effects of nuclear weapons under catch-all terms such as "emboldenment" while ignoring other potential effects of nuclear acquisition.