What one is left with, then, are two puzzles. The first is that while in many cases the United States has been able to actively shape its partners’ military contributions—even in seemingly unlikely cases where allies hosted considerable numbers of US troops—other times it has failed to do so. The second is that in some cases the United States has actually preferred that its allies not maximize their military capabilities, even when doing so could have allowed it to conserve its own resources.

The Argument in Brief

These puzzles raise the following question: Under what conditions is the United States willing and able to encourage allied burden-sharing? The answer lies in what I call alliance control theory. Alliance control theory predicts that patrons like the United States will calibrate their burden-sharing pressure toward allies in consideration of three factors—an ally’s latent military potential, the external threat environment, and the patron’s resource constraints. It shows that a substantial amount of variation in asymmetric alliance burden-sharing is the result of bargaining between the patron and its weaker allies. Although alliances can discourage allies from investing in their own defense, a patron can mitigate this tendency by combining assurances of support with threats of abandonment. The extent to which the patron’s protection can be made conditional on allies’ burden-sharing efforts, in turn, depends on whether the patron is both willing to ask them to contribute more and able to credibly threaten to abandon them if they do not.

In deciding to seek allied contributions, a patron must balance competing priorities. On the one hand, burden-sharing allows a patron to secure military power for collective defense against shared adversaries without bearing the costs of doing so itself. For this reason, patrons can be expected to encourage allied burden-sharing when the alliance’s external threat environment is severe and when their own resources are constrained. On the other hand, asking allies to shoulder more responsibility for defending themselves reduces both the value of the alliance to them and their dependence on the patron’s protection. As such, patrons are likely to tailor their efforts at securing allied contributions not only to maximize cost savings, but also to minimize the risk of empowering allies to go their own way and exit the alliance. Patrons, in other words, are likely to take a “Goldilocks” approach to allied burden-sharing. While the smallest allies have little to contribute, and so pose the least risk of defection, larger allies, who have more to contribute, pose the greatest risks, since they have greater potential to fend for themselves outside of the alliance. Counterintuitively, then, it is not the allies who have the greatest potential to provide resources for collective defense that face the most pressure to contribute. Rather, it is moderately sized allies who are strong enough to make meaningful contributions, but not so strong that they can choose to leave the alliance given sufficient investments in defense.

If a patron does choose to seek greater burden-sharing, its success then depends on whether allies fear being abandoned by it. Two factors shape the effectiveness of patron burden-sharing pressure: whether a patron’s threat of abandonment is credible, and how badly allies need protection. A patron can more believably threaten to walk away from its alliances when it faces strains on its resources and pressure to retrench from domestic actors, which constrain its ability to maintain its commitments. Its threat of abandonment is also likely to carry more weight for allies who perceive a greater level of external threat and thus can less easily afford to lose the patron’s protection.

Patrons, in sum, face a burden-sharing dilemma—they must balance discouraging free-riding and encouraging allies to remain loyal to the alliance through a reduction of allies’ incentives and capabilities to act independently. Stated differently, patrons in asymmetric alliances must find a trade-off between control—their ability to influence allies’ preferences and persuade them to act in ways that align with their own—and cost-sharing—their ability to reduce the costs of military readiness for themselves and shift those costs onto their partners. Although patrons can never fully overcome this dilemma, they can nevertheless mitigate it by making their protection conditional on allied burden-sharing and manage it by exercising caution in their attempts to secure burden-sharing.

Contributions of the Book

This book advances the study of alliance politics on several fronts. It shows, first, that existing literature underestimates great power patrons’ ability to persuade their allies to shoulder a greater portion of the collective defense burden. I demonstrate that great power patrons are often surprisingly capable of encouraging allies to contribute more, even when those allies receive considerable signals of support such as troop deployments. In the case of the United States, there are three reasons for this capability. First, US assurances are not distributed randomly, and many of the same factors that drive the need for reassurance—such as when allies doubt their patron’s reliability or perceive a high level of external threat—also make them predisposed to burden-share. Indeed, the very fact that allies need to be reassured suggests that they may be vulnerable to their patron’s pressure. Second, even if the United States provides signals of support, allies may nevertheless fear that that support could be withheld at a later date. Finally, the United States can in some cases use the possibility of reduced protection to motivate allies, pairing threats to punish an ally if it does not comply with assurances that the ally will not be punished if it does.

At the same time, my findings show that existing literature overestimates the degree to which patrons actually want their allies to contribute more. Often, the challenge of alliance burden-sharing is not a question of whether a patron can extract burden-sharing concessions from its allies, but of whether it is willing to tolerate the risks of doing so. The conventional wisdom on alliance burden-sharing, based on the logic of collective goods, suggests that larger partners in an alliance inevitably pay a disproportionate share of the alliance’s defense burden. Yet, when one considers burden-sharing in the broader context of the alliance and as an outcome of alliance bargaining, a central challenge for the patron becomes evident: allies who can contribute more can also do more for themselves and go their own way, pursuing policies that the patron may oppose and ultimately weakening the alliance. In other words, an ally’s capacity for self-reliance poses the risk of alliance abandonment—a possibility underplayed in existing literature, which tends to view an ally’s ability to find other partners as the primary cause of alliance abandonment, and which thus tends to underestimate the risk of alliance exit in bipolar and especially unipolar systems. Thus, if a patron is deliberately seeking to suppress its larger allies’ military capabilities for the purposes of maintaining influence over them, then the notion that larger allies must always or should always be encouraged to contribute disproportionately more of their resources toward the alliance’s common defense does not hold. In the final analysis, burden-sharing is not an unalloyed good, since it may result in diminishing returns for the collective good. Inequitable burden-sharing in US alliances should therefore not be seen as a predetermined result of collective goods logic that the United States perpetually resists, but rather as a conscious choice.

This book also bears broader implications for understanding alliance management while pointing to the limits of alliance treaties as commitment devices. Based on the assumption that the presence of a formal alliance treaty is one of the strongest assurances of support and indicators of friendly relations among states, the literature on alliances has long focused on understanding the causes and consequences of alliance formation. It has also emphasized how alliance treaty design—for example, whether a treaty imposes strict conditions on supports or limits on behavior, is bilateral or multilateral, and is vague or precise—enables allies to lock in their leverage over partners. But the formation of an alliance is only the starting point; a great deal of bargaining takes place after an alliance treaty is signed. Moreover, a treaty’s ability to actually constrain partners and lock in bargaining leverage is limited, no matter how foolproof its initial design, since partners’ interests, capabilities, and intentions, along with the external environment, can change in ways that make the alliance less useful or shift bargaining power among members. While this book does not discount the importance of studying alliance formation or alliance treaty design, it does show, through the lens of burden-sharing, that what happens after alliances are formed between great power patrons and their weaker protégés is equally important. And by paying more attention to burden-sharing rather than treaty design, the book demonstrates that patrons are able to maintain their bargaining leverage over the long term by willingly accepting or even encouraging a degree of free-riding.

Finally, this book contributes to our understanding of alliances as tools of restraint and control. Scholars have long argued that alliances can restrain allies from launching offensive wars—both against each other, by fostering transparency and building trust, and against third parties, by allowing partners to mediate and use the threat of withholding support to dissuade adventurous allies. But an understudied mechanism by which alliances can restrain and control partners is by reducing their incentives for independent military arming or engaging in security competition with their neighbors, thus ameliorating local security dilemmas that could otherwise foster conflict. This book explores this mechanism precisely by identifying the conditions under which great power patrons are more likely to seek a way to limit their allies’ military power.