The nuclear nonproliferation regime stands at a daunting crossroads. The nuclear powers have made little progress toward disarmament, a failure that looms over the regime. Efforts to curb the Iranian nuclear program have stalled, and global interest in nuclear proliferation continues to expand. With the Russo-Ukrainian War, U.S.-Russia arms control efforts have crumbled. Meanwhile, China's and North Korea's nuclear arsenals grow, and even the United Kingdom has announced it will soon increase its nuclear stockpiles. The nuclear taboo, too, is tested, as Russia and North Korea rattle their nuclear sabers and nuclear brinksmanship is tested in Ukraine.

Against this dangerous backdrop, there is no better time to examine how the nuclear nonproliferation regime came about in the first place—and to subsequently reflect on how it might be restored. Enter Rebecca Davis Gibbons's The Hegemon's Tool Kit, an ambitious volume that offers a holistic look at how U.S. influence has created, expanded, and maintained the Non-Proliferation Treaty (NPT). This timely book offers valuable insights into how the United States has forged the international nuclear order.

Gibbons argues that without the painstaking efforts of successive U.S. administrations, the nuclear nonproliferation regime as we know it would simply not exist. Indeed, she writes that “the regime needed the United States to become widespread” and should therefore be considered part of a U.S.-led international order (p. 3). In this way, Gibbons's understanding of the regime itself and her theory of its success departs from other explanations of regime adherence, such as those that highlight strategic selection or the shaping power of norms.

What role, then, has the United States played in the establishment and maintenance of the nuclear nonproliferation regime? Washington, Gibbons explains, has long managed the regime by applying a diverse policy toolkit to sway states of all stripes toward the regime. The choice of which policy tool to use is primarily determined by the level of the target state's “embeddedness” in the U.S.-led order. Gibbons defines embeddedness as occurring when a state's preferences and values align closely with those of the United States, in line with her interpretation of the order itself as a product of U.S. interests and policies.

Highly embedded states, Gibbons argues, can be swayed toward adherence by low-cost tools, such as bilateral and multilateral diplomacy. The holdouts—states that tend to align less with U.S. interests from the get-go—may instead be persuaded by more costly tools, such as high-level leadership, positive inducements, or coordinated pressure from the United States and its allies. Fully antagonistic states might be subject to coercion, such as threats of reduced aid or even threats to use force.

Gibbons tests these arguments with a series of case studies. She first investigates U.S. influence on decision-making in Japan, Indonesia, Egypt, and Cuba as they considered joining the NPT. Throughout these cases, Gibbons argues that it was U.S. influence—not security concerns, norms around fairness, or domestic economic considerations—that drove adherence. In this way, the book provides a robust test of various theories of regime adherence. The book then advances to explore how the regime was maintained. Gibbons chronicles the U.S. role in the 1995 debate over NPT extension; argues that the United States effectively coordinated with allies to secure the indefinite extension of the treaty; and, finally, demonstrates Washington's mixed success in promoting a modernized International Atomic Energy Agency safeguards system. The cases show both the breadth and depth of U.S. authority within the nonproliferation regime.

Although U.S. leadership has accomplished much in the nonproliferation space, it is worth recalling that both highly embedded states (such as Israel) and minimally embedded ones (such as North Korea) have failed to join or adhere to the NPT, despite efforts by the United States. Moreover, there are a handful of cases where Gibbons's typology imperfectly matches the historical record. For example, Gibbons expects coercion to be used only against states that are highly antagonistic toward the U.S.-led international order. Yet coercive threats to withdraw U.S. military support from South Korea in the 1970s were likely instrumental in Seoul's choice to join the NPT—even though the country was a valuable strategic ally to the United States at the time.

Gibbons argues that only a superpower with a sophisticated bureaucracy dedicated to foreign policy—as well as the resources and expertise to employ aid, trade, and influence in key global institutions—would be sufficiently well situated to oversee a project so large as crafting and safeguarding the nonproliferation regime. The flip side of this argument, then, is twofold. First, as cracks in the U.S. foreign policy toolkit steadily emerge, Washington's ability to induce and coerce states to adhere to the standards of the nonproliferation regime will also begin to fail, if it has not done so already. Second, Gibbons's theory suggests that elements of the nuclear nonproliferation regime that lack U.S. leadership will struggle to succeed because they will lack a superpower's ability to leverage a wide toolkit of policies. This bodes poorly for the Treaty on the Prohibition of Nuclear Weapons, which neither the United States nor any other nuclear power has signed.

At times, The Hegemon's Tool Kit underestimates the importance of other key actors. After all, the United States was hardly the only advocate for the nonproliferation regime. As Andrew Coe and Jane Vaynman argue, coordination between the United States and the Soviet Union was critical to expanding the regime among Soviet allies, including the Eastern Bloc countries, while research from J. Luis Rodríguez finds that leadership from nonnuclear states such as Mexico boosted the appeal of the “grand bargain.”

Nevertheless, the United States was undoubtedly a major player in the nonproliferation regime, and Gibbons's book demonstrates just how extensive U.S. influence has been throughout the years. In doing so, Gibbons offers an insightful narrative that highlights the significance of U.S. leadership at all stages of the development of the nuclear nonproliferation regime. As the nonproliferation regime struggles with the dangers of the modern nuclear order, will the United States still be able to rise to the challenge and provide the leadership that the regime seemingly demands? Gibbons warns that the answer may be no.