Why do great powers like the United States have trouble getting their smaller allies to take on a “fair share” of the alliance's security burdens? This question rose to the top of the U.S. foreign policy debate during the presidency of Donald J. Trump, who routinely and ostentatiously accused core American allies of underinvesting in their own defense. For many, existing international relations (IR) scholarship seemed to offer a plausible explanation for why such grievances appeared so intractable in modern alliance politics. “[O]nce an alliance treaty has been signed,” Mancur Olson and Richard Zeckhauser argued in a canonical article, “the larger powers are immediately deprived of their strongest bargaining weapon—the threat that they will not help to defend the recalcitrant smaller powers—in any negotiations about the sharing of the common burden” (“An Economic Theory of Alliances,” Review of Economics and Statistics, August 1966, 237). The implication was straightforward: as long as great powers were committed to the alliance's overall defense, they were likely to be given short shrift by smaller partners in their appeals for equitable burden-sharing.

Brian Blankenship's book offers a different way to think about the alliance burden-sharing question. Using meticulously documented case st