Primacy checks destabilizing proliferation—empirical analysis proves, accounts for their defense

Matthew Kroenig highlights a number of reasons why US policymakers seek to limit the spread of nuclear weapons: “Fear that nuclear proliferation might deter [US leaders] from using military intervention to pursue their interests, reduce the effectiveness of their coercive diplomacy, trigger regional instability, undermine their alliance structures, dissipate their strategic attention, and set off further nuclear proliferation within their sphere of influence.”8 These are not the only reasons for concern about nuclear proliferation; also notable are the enhanced prospects of nuclear accidents and the greater risk of leakage of nuclear material to terrorists.9 Do deep engagement’s security ties serve to contain the spread of nuclear weapons? The literature on the causes of proliferation is massive and faces challenges as great as any in international relations. With few cases to study, severe challenges in gathering evidence about inevitably secretive nuclear programs, and a large number of factors in play on both the demand and the supply sides, findings are decidedly mixed.10 Alliance relationships are just one piece of this complex puzzle, one that is hard to isolate from all the other factors in play. And empirical studies face the same selection bias problem just discussed: Nuclear powers are more likely to offer security guarantees to states confronting a serious threat and thus facing above-average incentives to acquire nuclear weapons. Indeed, alliance guarantees might be offered to states actively considering the nuclear option precisely in order to try to forestall that decision. Like a strong drug given only to very sick patients, alliances thus may have a powerful effect even if they sometimes fail to work as hoped. Bearing these challenges in mind, the most relevant findings that emerge from this literature are: • The most recent statistical analysis of the precise question at issue concludes that “security guarantees significantly reduce proliferation proclivity among their recipients.”11 In addition, states with such guarantees are less likely to export sensitive nuclear material and technology to other nonnuclear states.12 • Case study research underscores that the complexity of motivations for acquiring nuclear weapons cannot be reduced to security: domestic politics, economic interests, and prestige all matter.13 • Multiple independently conceived and executed recent case studies nonetheless reveal that security alliances help explain numerous allied decisions not to proliferate even when security is not always the main driver of leaders’ interest in a nuclear program.14 As Nuno Monteiro and Alexandre Debs stress, “States whose security goals are subsumed by their sponsors’ own aims have never acquired the bomb. … This finding highlights the role of U.S. security commitments in stymieing nuclear proliferation: U.S. protégés will only seek the bomb if they doubt U.S. protection of their core security goals.”15 • Multiple independently conceived and executed recent case research projects further unpack the conditions that decrease the likelihood of allied proliferation, centering on the credibility of the alliance commitment.16 In addition, in some cases of prevention failure, the alliances allow the patron to influence the ally’s nuclear program subsequently, decreasing further proliferation risks.17 • Security alliances lower the likelihood of proliferation cascades. To be sure, many predicted cascades did not occur.18 But security provision, mainly by the United States, is a key reason why. The most comprehensive statistical analysis finds that states are more likely to proliferate in response to neighbors when three conditions are met: (1) there is an intense security rivalry between the two countries; (2) the prospective proliferating state does not have a security guarantee from a nuclear-armed patron; and (3) the potential proliferator has the industrial and technical capacity to launch an indigenous nuclear program. In sum, as Monteiro and Debs note, “Despite grave concerns that more states would seek a nuclear deterrent to counter U.S. power preponderance,” in fact “the spread of nuclear weapons decelerated with the end of the Cold War in 1989.” Their research, as well as that of scores of scholars using multiple methods and representing many contrasting theoretical perspectives, shows that US security guarantees and the counter-proliferation policy deep engagement allows are a big part of the reason why.