Since the 2016 presidential election, Donald Trump has shaken some core principles of U.S. foreign policy. These include calling NATO obsolete and indicating that he would accept U.S. allies South Korea or Japan going nuclear. What role do such alliances play in stemming nuclear proliferation? This is the question that Alexander Lanoszka tackles in his rich and thoughtful book Atomic Assurance.

The book argues that alliances are “less useful than often presumed” in preventing proliferation, that alliance coercion in particular has played “less of a role in nuclear proliferation than some accounts suggest,” and that it is more difficult to reverse a program than to prevent its initiation. Instead, Lanoszka contends that conventional deployments are essential elements in reassuring allies and that economic and technological pressures are more effective means in stemming nuclear proliferation (p. 22).

Atomic Assurance begins with a presentation of its theoretical argument and of alternative theories, which point to the importance of U.S. nonproliferation efforts, security threats (the “adversary thesis”), the economic preferences of ruling coalitions (the “domestic politics thesis”), or the psychology of leaders and their national identity conception (what Lanoszka calls the “prestige thesis”) for preventing the spread of nuclear weapons. The book then reviews U.S. nonproliferation policies since 1945 before delving into three deep case studies (West Germany, Japan, and South Korea) and five shorter case studies (Great Britain, France, Norway, Australia, and Taiwan). It concludes with a discussion of policy implications. The book stands out for its concise and enlightened perspective on the foreign policy priorities and fiscal constraints of various U.S. administrations, as well as for the richness of the archival evidence in its three main case studies.

Even though the empirical work is excellent, Lanoszka does not always provide definitive evidence to support the book’s theoretical argument. Consider his discussion of Japan. He argues that U.S. assurances did not limit Japan’s interest in nuclear weapons and that President Richard Nixon was reluctant to use coercive tools to prevent Japan’s nuclear proliferation, even though he was worried about its realization, thus illustrating the challenge of coercing a nuclear ally to remain non-nuclear (pp. 79–80, 86, 102). An alternative argument would posit that Japan was never really intent on acquiring nuclear weapons. It briefly considered the option but decided against it, content to rely on U.S. assurances. If anything, Nixon had actually prodded Tokyo to think of acquiring a bomb as a way to reassure Beijing of the benefits of the continuing presence of U.S. troops in Japan.

Consider as well the discussion of West Germany. The book ultimately concludes that “alliance politics did inhibit what West Germany could achieve in its nuclear policy, but this pressure was much more diffuse and cannot be measured in terms of concrete coercive actions undertaken by the United States” (p. 76). Yet the Soviet Union made clear threats of military action if West Germany did not endorse the Non-Proliferation Treaty (NPT). The United States rallied to the Soviet position and explained its change in policy in meetings with West German officials, who felt coerced to remain non-nuclear in response to superpower complicity.

Finally, consider the discussion of South Korea. The book argues that concerns over U.S. assurances triggered South Korea’s nuclear weapons program, which was largely ended by U.S. coercive measures in the mid-1970s. This interpretation accords with the conventional wisdom, but it struggles to explain why South Korea’s interest in nuclear weapons lingered in the early 1980s. Departing from the conventional wisdom, Lanoszka argues that the Ford administration applied both coercion and reassurance, that U.S. threats were vague, and that Washington was mostly effective through its nonmilitary influence (esp. pp. 110–11, 121–25). This complex argument does not clearly explain why South Korea remained interested in nuclear weapons after U.S. coercive measures were applied in the mid-1970s. Alternatively, one could assign a greater role to U.S. assurances in the full arc of South Korea’s nuclear history. Not only did South Korea’s interest in nuclear weapons begin with concerns over U.S. assurances but it was also eliminated by the renewed assurances offered by the Reagan administration in the early 1980s.

Taken together, these last two cases raise some important questions about the meaning and nature of threats. What does an ally need to say to articulate a threat? When is a threat too vague to be effective? How do we know whether economic dependence is more effective than a coercive threat in inducing behavioral change?

It would be useful if the book offered some clear criteria for evaluating the effectiveness of different measures, but it seems more interested in expressing its theoretical claims as qualifications of existing theories. This approach raises the following questions. How effective does the literature presume alliances to be in stemming proliferation? How much of a role does it attribute to alliance coercion in stemming proliferation? And how much weight should be given to alliance politics if the book is correct?

Although the “domestic politics thesis” and the “prestige thesis” are robust theories of proliferation, offered by Etel Solingen and Jacques Hymans, respectively, the alternative argument—the “adversary thesis”—for preventing the spread of nuclear weapons is amorphous and does not reflect the range of views on the role of security and alliances in stemming proliferation.

Traditional security arguments suggest that states seek nuclear weapons to confront an adversary, but they also allow for some variation in the willingness to acquire nuclear weapons. Strong states may want to build their own weapons, but weak states may prefer to meet their security needs through an alliance with a nuclear power (Scott D. Sagan, “Why Do States Build Nuclear Weapons?” International Security 21[3], 1997, p. 57).

Similarly, arguments on the effects of alliances come in a variety of forms. The recent literature has highlighted a “dark side” of alliances, in which security patrons may coerce their protégés to remain non-nuclear. Yet it also offers some arguments on the relative effectiveness of coercive measures, instead of claiming that they are always effective. Ultimately, it would have been helpful if Lanoszka had engaged a more sophisticated version of existing arguments on the role of security and alliances throughout the book’s case studies, while making sure that its theoretical claims are articulated as explanations of empirical patterns, rather than as amendments of existing arguments.

Despite these criticisms, Atomic Assurance is a significant contribution to our understanding of the effect of alliances on nuclear proliferation. Now more than ever, this is a subject of prime academic and policy importance.