Title: Promises under Pressure
Description: Great power patrons frequently reassure allies of their protection, whether by stationing troops abroad, visiting allied countries, or making public statements. In the case of the United States, observers and practitioners alike have emphasized the need to instill confidence in U.S. allies. However, allied reassurance is fundamentally puzzling because it gives away a key source of bargaining leverage: the threat of abandonment. Patrons should ideally strive to limit the extent to which they are perceived as committed to allies, lest they encourage allies to free-ride on their protection and contribute little to the common defense. Existing literature tends to either treat reassurance as a secondary effect of deterrence, or to focus on understanding how patrons can reassure their allies rather than why. Studies that do provide explanations for reassurance, for their part, often regard reassurance as strategically suboptimal, and emphasize domestic political factors that drive reassurance.

The causes of reassurance are thus poorly understood. I argue that although reassurance can have adverse consequences, patrons have incentives to reassure to the extent that allies have the capacity to exit the alliance. The more credible an ally’s threat to pursue outside options, and the more costs that doing so would impose on the patron, the more reassurance it will receive. Patrons thus face a dilemma, trading off between withholding reassurance to drive hard bargains with allies and reassuring allies to dissuade them from exiting the alliance. This dilemma may be mitigated, however, if a patron can make its assurances conditional on allied burden-sharing by combining its assurances with threats of abandonment. These threats are more potent to the extent that a patron faces domestic pressure to retrench from its foreign commitments, and that allies face severe threat environments. I test the theory using a mixed-method approach that combines statistical analysis of an original dataset on American reassurance and allied burden-sharing between 1950 and 2010 with qualitative historical case studies.

In Chapters 1 and 2, I introduce the concepts of alliance reassurance and burden-sharing and review the literature on both concepts. I argue that reassurance is puzzling in light of existing theories of alliance bargaining which stress the threat of abandonment as a source of leverage. The “reassurance dilemma” that patrons face, however, is that withholding reassurance may encourage allies to distance themselves from the alliance and seek outside options. In Chapter 3, I present a theory of bargaining leverage in asymmetric alliances in order to identify the conditions under which this dilemma is most severe—and thus to explain variation in patron reassurance and allied burden-sharing. I posit that reassurance serves the purpose of discouraging allies from leaving the alliance; the more credible allies’ threats of exit, the more reassurance they will receive. However, patrons can make their assurances conditional on allies’ burden-sharing efforts if their own threat of exiting the alliance is credible as well.

I present a simple formal model illustrating both the tradeoffs between reassurance and burden-sharing, as well as the conditions under which patrons are more likely to reassure and allies are more likely to increase their contributions to the alliance. I then introduce hypotheses for testing the theory’s observable implications. Chapter 4 presents the quantitative analysis on the determinants of patron reassurance and allied burden-sharing. First, using an original dataset of U.S. reassurance collected and analyzed with automated text analysis, I use statistical models to identify correlates of U.S. willingness to offer reassurances. Second, I study allied burden-sharing using data on allies’ military spending, support for U.S. military bases, and participation in U.S. foreign military interventions. The quantitative findings strongly support the theory; the United States reassures allies that are at greater risk of exiting the alliance more, while allies more dependent on U.S. protection also spend more on defense, provide more compensation for the costs of U.S. military bases, and participate in U.S. foreign military interventions at a greater rate.

In Chapters 5-8, I conduct case studies on U.S. reassurance and burden-sharing pressure toward West Germany, Japan, South Korea, and the United Kingdom during the 1960s and 1970s. Process-tracing of these cases shows that the United States saw reassurance as a way of discouraging its allies from pursuing outside options—in particular nuclear weapons and rapprochement with the Soviet Union. However, the United States was simultaneously able to extract significant burden-sharing efforts, especially from West Germany and South Korea owing to their geographic vulnerability, and during the early 1970s due to doubts about U.S. reliability in the aftermath of the Vietnam War. Finally, Chapter 9 concludes with a summary of the analysis, as well as a discussion of implications and avenues for future research. My findings suggest that by withholding reassurance and deliberately casting doubt on its protection, a patron makes its allies prone to reconsidering their reliance on it and to instead pursue outside options.