The trajectory of twentieth-century history could have been radically altered if either of two high-stakes military alliance negotiations, the first conducted in 1901 and the other in 1939, had resulted in success rather than failure. Had Britain and Germany struck an alliance in 1901, Europe’s great powers might not have subsequently plunged into World War I. Had Britain, France, and the Soviet Union reached agreement four decades later, they might have forestalled the outbreak of World War II. In Arguing about Alliances, Paul Poast contributes significantly to the study of alliances by remedying the glaring lack of attention that international relations (IR) scholars have paid to these and many other incipient alliances that were negotiated but never ultimately concluded, a crucial international security analog to Sherlock Holmes’s “dog that didn’t bark.”

Poast’s theory of alliance negotiations posits that agreement hinges primarily on the compatibility of the negotiating parties’ war plans and secondarily on the parties’ ability to pursue alternative security strategies. If the states possess compatible war plans, meaning that they agree both on the principal threat against which an alliance should be directed (i.e., strategic compatibility) and whether that threat should be countered via an offensive or defensive military doctrine (i.e., operational compatibility), then the theory predicts that they will successfully ally. Conversely, if their plans are incompatible, the existence of attractive outside options—construed as a state’s ability to secure itself via unilateral action, buck-passing, or striking an alliance with different partners—becomes salient. If all of the parties possess such an option, then they should fail to reach an alliance, but if only some do, the outcome of negotiations will be indeterminate.

Poast employs multiple methods to test the two hypotheses that war plan compatibility will be substantially more likely than incompatibility to result in alliance agreement, and that the availability of outside options will be more salient to the outcome of negotiations when war plans are incompatible. He pits these propositions against the alternative explanations that agreement hinges on the degree to which the parties either fear becoming entrapped in war by prospective partners or believe that prospective partners are insufficiently powerful or reliable. A series of statistical analyses of 197 alliance treaty negotiations among European states spanning the years 1815–1945 provides robust support for both propositions: Poast’s fully developed model predicts the outcome of alliance negotiations in 74% of the cases.

These findings are complemented by the author’s examination of two signal historical cases, the aborted alliance negotiations between Britain and Germany in 1901 and the successful multilateral discussions that produced the North Atlantic Treaty Organization in 1948–49. In the Anglo–German case, although both states feared Russia, London perceived the Russian threat to be most acute in Asia, whereas Berlin perceived it to be most worrisome in Europe, and Britain alone possessed outside options; namely, the pursuit of “splendid isolation” in Europe and establishment of an anti-Russian alliance in Asia with Japan. Poast demonstrates that British leaders abandoned the negotiations for these reasons and not because they harbored concerns about Germany’s reliability. Transatlantic alliance discussions in the late 1940s were similarly characterized by war plan incompatibility: the United States and United Kingdom were most concerned about the Soviet threat to northern Europe, whereas France was preoccupied by the Soviet danger to southern Europe, as well as by the availability to the United States of the outside option of containing the Soviets unilaterally. Poast similarly shows that these factors, not US entrapment concerns or worries about its European counterparts’ reliability, played the dominant role in influencing the course of the talks.

Arguing about Alliances distinguishes itself from a flood of recent contributions to the literature on alliances not only by dint of its unique focus on alliance negotiations, but also because of its exceptionally sophisticated research design. Poast devotes the book’s entire second chapter to explaining his meticulous coding of the central concepts of alliance negotiations, alliance negotiation outcomes, strategic compatibility, and operational compatibility. Next, he subjects the first hypothesis linking war plan compatibility with alliance negotiation outcomes to cross-tabulation analysis, followed by a series of five increasingly complex statistical models, which layer on 11 control variables. He proceeds to conduct additional regressions that add more control variables into the mix and introduces alternative measures of control variables employed in the previous models. He then subjects these results to a battery of sensitivity tests to show that they are not compromised by deviations from the assumptions that he employed in creating his original dataset of 91 failed European alliance negotiations between 1815 and 1945. This is followed by Poast’s use of both cross-tabulations and propensity-score matching to demonstrate that the existence of an outside option, in the form of a state’s membership in an extant alliance, is likely to affect the outcome of negotiations only when the parties’ war plans are incompatible. Finally, he carefully process-traces the Anglo–German and NATO cases to both illuminate the causal pathways by which his independent variables produced the outcome of alliance negotiations and to rule out alternative explanations.

The most impressive works in social science generate at least as many questions as they answer, and Arguing about Alliances is no exception. First, the book begs the question of where alliance war plans come from. Although its chief contention that states will be most likely to conclude alliances if they agree on the threat at issue and how best to confront it is strongly validated, this finding is hardly shocking and inevitably directs attention to the preceding link in the causal chain. The theory would be even more compelling if it could identify the most important international systemic, domestic political, or individual-level variable(s) involved in the construction of national war plans. Presumably, this task would necessitate anchoring Poast’s free-floating theory to a larger research program in IR.

Consideration of the determinants of war plans would also help explain the extent to which political leaders can modify them to facilitate alliance agreement. Revealingly, Poast indicates that, in both of his case studies, the diplomatic acumen of the alliance negotiators proved central to the outcome, and he thereby insinuates that they enjoyed extremely wide leverage in being able to adjust their war plans. Whereas he notes that Germany “did not moderate its plans” (p. 133) in 1901 because its leaders stubbornly clung to the erroneous belief that Britain lacked outside options, the success of the 1948–49 transatlantic negotiations was attributable to “the negotiating creativity of the participants” (p. 168). In light of Poast’s apparent privileging of decision-making variables over international systemic ones in the alliance-building process, it is odd that he refrains from advancing any concrete policy implications in the book’s conclusion.

A second question also pertains to the possible extension of Poast’s theory, but on the other end of the causal chain: Can his theory of alliance formation also explain alliance dissolution? It is plausible that the same factors that Poast introduces to explain why alliances are created in the first place also account for the manner in which they end. Conceivably, some alliances die a natural death by persevering through to the fulfillment of their strategic objective, whereas others expire prematurely. The unnatural deaths suffered by some alliances may well be because members’ previously compatible war plans began to diverge even as some or all of their members held viable outside options. The early twentieth-century alliance between Britain and Japan (1902–21) and the Cold War–era US Mutual Security Treaty with Taiwan, the Southeast Asian Treaty Organization, and the Central Treaty Organization represent potentially instructive cases on this score.

A third question addresses the logic underpinning the theory: Why must compatible war plans always trump outside options? If the war plans of states X and Y are identical but X has the alternative option of passing the buck to Y (and possibly others), a rational X will almost certainly prefer to buck-pass and thereby avoid the costs of countering the shared enemy than conclude an alliance and share those costs. For example, although Poast refers to the joint global war plan secretly agreed to by high-level US and British military officers in March 1941 as an example of war plan compatibility leading to alliance agreement (pp. 34–36), he refrains from noting that, after March 1941, the Roosevelt administration continued passing the buck to London and, after Germany’s invasion of the USSR in June, to Moscow as well. It did not conclude a formal alliance with both countries until January 1942, a month after the attack on Pearl Harbor and Hitler’s declaration of war against the United States. This behavior appears to cast at least some doubt on the theory’s contention that the viability of outside options only “comes into play” (p. 170) when the compatibility of parties’ war plans is low.

In sum, Arguing about Alliances is an innovative and breathtakingly rigorous work that adds considerably to scholars’ understanding of alliances, even as it helpfully generates a range of questions to guide future research on the subject.