A recent US president once complained that Washington’s allies do not pay their fair share for defense. “Free riders aggravate me,” he stated bitterly. “You have to pay your fair share.”

This president was not Donald Trump, who repeatedly made headlines by casting doubt on his willingness to protect US allies unless they made sufficient defense contributions.  Rather, it was his predecessor, Barack Obama. Indeed, Obama’s Secretary of Defense went so far as to warn US partners that “there will be dwindling appetite and patience…to expend increasingly precious funds on behalf of nations that are apparently unwilling to devote the necessary resources…to be serious and capable partners in their own defense.”

Obama and Trump were hardly the first US leaders to solicit greater burden-sharing from US allies. In late 1953, President Eisenhower’s Secretary of State John Foster Dulles warned that unless Europe became more self-reliant, the United States would undertake “an agonizing reappraisal of basic United States policy” toward its commitment to the Continent. In much the same way, John F. Kennedy threatened to withdraw US troops from West Germany to secure compensation for the costs of stationing them in the country, declaring that “we cannot continue to pay for the military protection of Europe while the NATO states are not paying their fair share and [are] living off the ‘fat of the land.”’ Across numerous administrations, the need for allied burden-sharing has been a constant refrain in US foreign policy.

But there is striking variation in the success and, in fact, the incidence of US burden-sharing pressure. In some cases, burden-sharing pressure efforts succeed—Jimmy Carter, for instance, persuaded South Korea to increase its defense spending to 6 percent of gross national product when he leveraged the possibility of troop withdrawals. At other times, they fail—US pressure on the United Kingdom to retain its military presence in Asia during the 1960s amounted to little.  And then there are those cases, perhaps most puzzling of all, where US officials have notably declined to seek greater allied contributions to the common defense. During the early 1970s, for example, the Nixon administration refrained from seeking a substantially greater Japanese military role despite the explicit ambition of the Nixon Doctrine to delegate more responsibility to allies to defend themselves. Similarly, US policymakers have repeatedly balked at the prospect of a united, independent European defense capability since the end of the Cold War.

This book attempts to understand such variation in burden-sharing within US alliances. It investigates why the United States sometimes puts pressure on its allies to bear more of the collective burden (and sometimes doesn’t) and when these burden-sharing pressures succeed (or don’t). To that end, it advances two central claims. First, US decisions to encourage allied burden-sharing at all are the product of calculations about both the benefits and the risks of greater allied self-reliance. Burden-sharing is useful insofar as it can purchase a similar amount of collective military power at lower US cost. But in some cases—particularly those in which allies have a realistic capability to go their own way—the United States actually prefers that its allies not assume more responsibility for their own defense, since doing so can reduce their dependence on US protection and, by extension, US influence. Second, if the United States elects to encourage allied burden-sharing, its success depends on whether allies fear that it will abandon them. The more credible its threat of abandonment, and the more allies depend on its protection, the more successful it will be.

Each claim challenges strands of conventional wisdom on burden-sharing. The first rethinks the assumption that larger allies disproportionately contribute to collective defense and are always encouraged to do so.  I suggest that patrons like the United States actually have good reason not to encourage their largest allies to spend more on defense—namely, to prevent them from becoming too independent. The second claim confronts the notion that the United States’ ability to pressure allies into greater burden-sharing is inherently precluded by its disproportionately great power and vast network of overseas troop deployments. I show that the United States is in fact sometimes able to wield the threat of abandonment to encourage greater burden-sharing even among allies that host a substantial US troop presence. Thus, even though great powers like the United States may be constrained in their ability to solicit allied military contributions, they are far from helpless in doing so. The operative constraint on alliance burden-sharing is in some cases not the United States’ ability to secure it, but simply its willingness to seek it.

What We Know (and Don’t Know) about Burden-Sharing

Burden-sharing refers broadly to actions by which an alliance member contributes to the alliance’s capacity to carry out its objectives. These contributions can take a number of military or nonmilitary forms, such as deploying forces to active conflicts, hosting military bases, providing logistical support, or supplying aid. In this book, I focus on burden-sharing in the form of allies’ efforts to provide for their own defense and enhance their own military capabilities.

Burden-sharing is core to the functioning of alliances. By enabling states to pool resources, burden-sharing crucially ensures that alliances can achieve their collective goals, as they are unlikely to succeed in deterring and defeating adversaries unless their members possess sufficient capabilities. A lack of burden-sharing, or free-riding, can cause an alliance to fail to achieve its objectives if the patron is unwilling or unable to contribute enough by itself, and it can create discord in the alliance by generating resentment among members who feel taken advantage of. Burden-sharing additionally facilitates the self-preservation of great powers. Scholars have long argued that overinvesting in military power sows the seeds of great powers’ long-term decline by diverting resources from more productive investments and innovation. They offer evidence that even financing military spending through debt does not avoid painful fiscal trade-offs, as doing so can cause inflation and contribute to economic crises. These problems are especially challenging in asymmetric alliances between a more powerful great power patron and weaker partners, as the patron’s greater capacity to provide security for the alliance makes it tempting for weaker partners to seek a free ride.  Securing allied burden-sharing thus allows a patron to ensure that its commitments do not exceed its resources, and that adversaries can be deterred at a sustainable cost.

The literature on burden-sharing, however, is generally pessimistic about its prospects in asymmetric alliances, largely portraying burden-sharing by smaller allies as an objective that great power patrons seek but have difficulty attaining. This pessimism partly has to do with the predominant view that alliances between great powers and weaker states feature an asymmetric exchange, wherein the former provides security for the alliance while the latter gives up some degree of foreign policy autonomy. From this perspective, the capacity for burden-sharing in asymmetric alliances is limited almost by definition; the great power already tacitly agrees to the costs of providing security in exchange for weaker states’ aligning their foreign policies with its own. Another cause for pessimism has to do with an understanding of alliances based on the logic of public goods, which emphasizes that larger alliance members tend to contribute disproportionately more to collective defense since it is their contributions that ultimately matter most. From this perspective, smaller allies rationally free-ride, and alliances with more members tend to feature more free-riding. Others likewise suggest that the global US military footprint and other assurances of US protection encourage allies to free-ride.

These studies thus leave gaps in our understanding of variation in burden-sharing. They cannot fully explain why France withdrew from NATO’s military command in 1966, despite being among NATO’s largest members; why Germany, since the end of the Cold War, has been among NATO’s lowest spenders on defense as a percentage of its gross domestic product (GDP); or why, by contrast, comparably smaller NATO members in the 2010s—notably Estonia, Latvia, Lithuania, and Poland—have punched above their weight. Moreover, they cannot fully explain why allies who hosted many US troops nevertheless spent a great deal on defense. Empirical research on the subject has produced mixed results and does not conclusively show that signals of alliance commitment negatively impact burden-sharing—some studies find that the presence of US troops has a negative effect on burden-sharing, but others find no such effect.  While there are cases in which US protection almost certainly discouraged allies from making significant efforts toward self-defense—perhaps most famously Japan, whose constitution’s Article 9 imposes restrictions on its military, and Iceland, whose membership in NATO came with the assurance that it would not need to have a military—there are yet other cases in which allies hosting tens or hundreds of thousands of US troops still contributed a great deal to their own security, such as West Germany and South Korea.

Existing literature likewise has difficulty explaining why the United States has in other cases been reluctant to seek more burden-sharing from its partners. In addition to seeking just a modest military contribution from Japan, US officials only grudgingly accepted the need for West German rearmament during the early Cold War so that it could assist NATO in counterbalancing Soviet power in Central Europe. Similarly, US policymakers were lukewarm about European proposals for a more united European defense policy during the 1990s and 2000s, and even the Trump administration balked at proposals for a European defense fund.