With the rise of irregular warfare and the onset of the global war on terror, the study of alliance politics became almost passé; theorizing about alliances was, like NATO, a Cold War relic. No more. From questions over America’s continued commitment to the North Atlantic Treaty, to concerns over US support for Saudi Arabia’s military campaign in Yemen, to the central role of alliance relations (via military aid to Ukraine) in the impeachment of President Donald Trump, the concepts of allies and alliances are more salient than ever. As a result, it is vital that scholars think critically about the meaning and purpose of alliances. The current state of international affairs shows that there remains much for us to learn about relations between allies and that doing so requires serious scholarly engagement.

That is why I am grateful to Evan Resnick for writing his book and for taking the time to thoroughly review my own book. I am pleased (thrilled, actually) that Resnick acknowledges the book’s relevance and labels it “innovative and breathtakingly rigorous.” But he does raise three important questions about the book, especially regarding the theory.

First, Resnick asks about the determinants of a state’s war plans: From where do they come? My theory holds that since joint war planning is a central feature of alliance treaty negotiations, such negotiations are more likely to end in agreement if the states have highly compatible ideal war plans. I treat a state’s ideal war plan as a given. I did not thoroughly discuss the origins of individual state war plans, because I wished to focus on the mechanics of the negotiations themselves. But Resnick rightly wonders about their origins: Could studying the international systemic, domestic political, or individual-level variable(s) involved in the construction of national war plans produce further insights into which states are likely to ally? I believe the answer is yes, and as Resnick points out, such work would be a useful extension on my theory.

Second, Resnick wonders why I privilege “war plan compatibility” over “outside options.” In particular, he notes how the United States continued to buck-pass to the British (and later, the Soviets) after military officers reached a secret global war plan in March 1941. I think this is simply a matter of timing, something that my theory is too coarse to explain. My theory holds that since the United States and British had compatible ideal war plans, they were highly likely to reach agreement (which they did). Moreover, I maintain that both the United States and British did not have attractive outside options. This should also have contributed to agreement (indeed, the combination of ideal war plan compatibility and lack of attractive outside options is why I label this a “Same Page” negotiation). Although it is true that the United States did not formally enter the war for another nine months, the British and Americans did sign the Atlantic Charter in August 1941, and US military aid to the British and Soviets, via Lend-Lease, expanded throughout the year.

Third, I completely agree that my theory could have implications for understanding the dissolution of alliances. Although I intentionally tabled postsigning considerations—such as ally commitments to the alliance or the eventual dissolution of the alliance—my theory would hold that divergence in ideal war plans (perhaps through the emergence of a new threat to one alliance member that is not shared by the other) or the appearance of a previously unavailable outside option (perhaps a technological innovation that now enables a state to take unilateral action) could, at minimum, prompt renegotiation of the alliance’s terms. At maximum, such changes could lead to the full collapse of the alliance. I hint at this possibility in the conclusion, but it is again an item worthy of future work.