Joshua Kertzer’s important and impressive new book begins with the observation that the concept of resolve is frequently used as an explanation for international events. The idea of resolve is particularly prevalent in the international security literature, as it is used to explain deterrence success or failure and the outcome of crisis bargaining and military conflicts. However, as Kertzer points out in Resolve in International Politics, “resolve” has neither been clearly defined nor carefully studied. The rational choice literature has defined resolve in differing ways, some of which overlap with other concepts, such as military capabilities. In the broader scholarly and public dialogue, resolve is often not defined at all, but is used as a “catchall residual” (p. 3) explanation for international outcomes when other explanations fail.

Clearly, the differing and sometimes casual uses of the word “resolve” make it difficult to study the concept using social science methods. Therefore, Kertzer does the discipline a great service by delving into the details of what resolve is and where it comes from. His basic definition of resolve as “firmness or steadfastness of purpose” (p. 8) is simple and follows the Oxford English Dictionary. However, he ultimately goes far beyond this by offering a comprehensive account of the factors that contribute to resolve.

Kertzer begins his discussion of the sources of resolve by noting that resolve can be viewed as a dispositional attribute, stemming from an actor’s character, or as a situational attribute, driven by the incentives and costs that an actor faces. Focusing specifically on resolve in the context of military interventions, he proposes an interactionist theory of resolve that incorporates both situational and dispositional factors. The situational factors that affect resolve are the costs of fighting and the costs of backing down, which include both reputational costs and the intrinsic interests at stake. The dispositional factors are time preference, risk preference, honor orientation, and trait self-control. By incorporating both situational and dispositional factors into his theory, Kertzer is able to offer insight into how dispositional attributes moderate the effect of situational incentives.

The empirical section of the book has two main parts. The first part is concerned with testing the author’s theory of the factors that influence resolve and uses resolve as the dependent variable. Kertzer begins with a laboratory experiment administered to university students. The participants were presented with a scenario about a hypothetical military intervention and asked their opinion about whether to initiate the intervention and then whether to continue it. In the experiment, he manipulated various situational factors and also collected information on participants’ dispositional attributes. In addition, he presents the results of a similar survey experiment that he performed using a national sample. Across both the laboratory experiment and the survey experiment, Kertzer finds that situational factors do affect resolve, but the effect of dispositional factors on resolve is even stronger and more consistent.

The second part of the empirical section focuses on how resolve impacts the outcome of military interventions by great powers. In this portion of the book, resolve is the independent variable. Having already established which factors are sources of resolve, Kertzer is in a good position to measure resolve without resorting to tautology. Using a Boolean logit model, he incorporates all of the situational and distributional factors discussed previously. Due to limitations on the information available, rougher measures of these concepts are employed in this part of the analysis than in the experimental part. Nonetheless, Kertzer’s measurement choices are plausible, and he employs multiple measures of the same concept so that the results do not depend on a single measurement choice. The key takeaway from this second part of the empirical analysis is that resolve does increase the probability of victory. In particular, Kertzer finds that reputational costs for backing down, as well as leaders’ time preferences and honor orientations, contribute to the probability of victory. As he himself notes, the apparently smaller impact of some of the other variables could be due to less appropriate measures (p. 139).

On the whole, this book makes several groundbreaking contributions. It provides the most thorough discussion to date about how to conceptualize resolve, it provides evidence of which dispositional and situational factors contribute to resolve, and it provides the strongest evidence yet that resolve itself influences conflict outcomes. The ultimate finding—that resolve matters—is in keeping with common belief, but we can be much more confident in this belief thanks to Kertzer’s work. Furthermore, the analysis of the sources of resolve that he provides lays the groundwork for more social-scientific analysis of this concept in the future. The book has implications for a variety of literatures, but in the interest of space I focus on two.

One obvious body of scholarly work to discuss is the rational choice literature. Kertzer does not frame his work as being entirely opposed to the rational choice perspective, but he is critical of rational choice models for conceptualizing resolve simplistically. He is particularly critical of the choice to conceptualize resolve as a low cost of war (pp. 5, 144), since he demonstrates that dispositional factors matter more than the costs of war and can even influence how the costs of war are interpreted. On the one hand, Kertzer is correct that some rational choice models could perhaps benefit from incorporating a more nuanced conceptualization of resolve. Some dispositional traits, such as time and risk preferences, could easily be defined in formal terms. On the other hand, formal modelers will have to weigh the benefits of incorporating his findings against the downsides of more complicated models. Even if it is not entirely realistic, a definition of resolve as a low cost of war or a high value placed on the issue at stake might still be justifiable for simplicity’s sake, based on the argument that resolved actors behave approximately as though they have a low cost of war or high valuation of the stakes. Whether it is necessary to incorporate a more nuanced and dispositional conceptualization of resolve into a formal model may depend on the research question.

As a scholar who studies international signaling and credibility, I want to draw out some implications for the signaling literature as well. Kertzer points out that some of the factors influencing resolve are observable to external actors, but other dispositional sources of resolve are difficult to observe externally. (Indeed, the author states that actors may not even fully know their own level of resolve [p. 148], although I would argue that they are likely to know much more about it than external observers.) If resolve is difficult to observe, this raises the question of exactly how it influences outcomes. My book argues that this happens through statements and other signals of resolve, but there may also be other mechanisms. There is a great opportunity for future research to bring together Kertzer’s work on the nature of resolve itself and work on signaling resolve. By coding resolve itself using Kertzer’s methods and coding signals of resolve using methods proposed by myself or other scholars, it might be possible to directly test whether inherently more resolved states actually signal more resolve and whether resolve itself or signals of resolve contribute more to influencing dispute outcomes.

Although this book is highly impressive, one aspect is potentially controversial. Kertzer asserts that resolve is both situational and dispositional because he views resolve as a state of being rather than a character trait. As he notes, this is an ontological claim (p. 165), which is difficult to either prove or refute. However, this way of conceptualizing resolve might not be the most satisfying or the most useful for future research. If there are some aspects of resolve that change with the situation and other aspects that remain constant, it becomes difficult to consider resolve as a unified concept. Thus, it might be more satisfying to refer to constant dispositional factors as resolve and to call situational factors by another name. We should also be careful about grouping dispositional and situational factors together for empirical analysis, since they may have different effects. As Kertzer notes, a unified dispositional measure of resolve could in principle be created by country–year, but a situational measure would have to be created for each individual dispute (p. 151). However, regardless of whether situational factors are considered part of resolve, I fully agree with him that it is important to explore their impact, and so this is not a major criticism.

Overall, Kertzer’s theoretically innovative and methodologically sophisticated book greatly improves our understanding of the crucial but underexplored concept of resolve. It is likely to be very influential in the field of international security and should be on the reading list of all scholars who study conflict bargaining, as well as those who take a behavioral approach. I also recommend the book to scholars who work in other areas of political science, since resolve can be relevant in economic as well as security-related interactions, and also in domestic interactions as well as in interactions between states.