The recent calls within conflict studies to pay more attention to peace is perhaps emblematic of the contemporary era where despite the lack of militarized interstate disputes, let alone wars, many interstate relations are not at the highest levels of peace. This by itself is enough to make a contribution on peace by leading conflict scholars deserving of attention, but The Puzzle of Peace is much more than just another plea to go beyond the study of peace-as-absence-of-war.

While Gary Goertz, Paul F. Diehl, and Alexandru Balas prefer “positive peace” to “stable peace,” their approach is closer to the stable peace approach of Kenneth Boulding or Karl Deutsch than that of Johan Galtung. In Part I, the authors share their conceptualization of peace, develop a measure for their peace scale, and then prepare a data set on peaceful change. They develop a five-level peace scale composed of rivalry (further divided into severe and less rivalry), negative peace, and positive peace (further divided into warm peace and security community). A data set then delineates the peace levels for pairs of states that have relationships (based on several criteria, such as contiguity, strong regional economic integration, and rivalry) and thus allows for the analyses of transitions across levels of peace from 1900 onward. This sort of approach, which sees peace as multilayered and multilevel, is mainly missing in the quantitative conflict literature, and I expect the book to spark more interest in this approach within the subfield. While such frameworks are more common in the (mainly comparative case–based) stable peace literature, the levels and indicators here are particularly well articulated and defended. Overall, while this is not the first or only theoretical framework or data set on peaceful levels, it is highly deserving of attention.

Given the past work of Diehl and Goertz, it comes as no surprise that rivalry and territory are central here. Since the rivalry literature has been prolific over the years, the connection to rivalry will hopefully result in more scholars of rivalry thinking about what peaceful relationships entail. While I have disagreements with the authors on some points relating to the peace scale (such as that a relationship can be both in negative peace and in rivalry, or that negative peace could have benefited from being divided into two), their approach is consistent with their logic. A lot of work went into thinking about the scale (e.g., they avoid turning negative peace into the residual of everything that is not included in the ends of the scale), as well as determining the placement of the cases (e.g., the authors rely upon several techniques when considering how pairs are to be grouped, including using anchor cases). Overall, the attention that the authors show to conceptualization, definition, and measurement issues can be useful for classroom instruction.

Besides their work with the theoretical framework and the data set, Goertz, Diehl, and Balas argue and demonstrate that the decline of violence thesis (associated most prominently today with Stephen Pinker, whom the authors are in particular addressing here as the book starts and concludes with references to him) gives a limited understanding of whether and how our “better angels” are operating, and that a peace-as-absence-of-violence will not suffice. By demonstrating that interstate relationships experienced greater amounts of positive peace and less rivalry in the post–World War II era, they are showing that the decline in violence thesis is an understatement as there is a separate increase in peace.

Disagreements over territorial control have played a major role in past interstate wars. The authors argue that what has changed is that norms after World War II prevent transfers based on force (p. 12). This “territorial integrity principle” is maintained by an interlinking set of institutions, state practice, and international law. Whereas territory has received attention in both the quantitative conflict and stable peace literatures, the authors offer new insights to these debates by showing how various features, that is, uti possidetis, and conflict management techniques, are connected to each other and help to sustain this system. The authors here are setting a challenge for future researchers as they point out that explanations of territory help better to explain the transitions away from the lowest levels of peace (p. 18). For those who believe that territory is also important for reaching the highest levels of peace, this suggests the need to further unpack territory. For others, the challenge will be to show that a non-territory-based explanation can “outperform” one based around territory.

Chapters in Part II engage with a variety of topics to highlight both the extent of the change in norms and the reasons for the changes. These chapters could at times engage more with the authors’ peace scale (e.g., the maritime boundaries chapter hardly mentions it) and with the existent stable peace literature, which they engage with mainly in the conceptual discussions of peace in Part I, and not in terms of its arguments or findings. Nonetheless, throughout the chapters in Part II, they succeed in demonstrating that during the twentieth century, there was a qualitative difference in understandings of peacefulness through their analyses of decolonization and state recognition, norm against conquest, norms against secession, territorial change, conflict management techniques, international governmental organizations and international courts, and maritime disputes. These are all important topics on their own, and the relevant chapters should be of interest to scholars working on those topics even if they have no interest in peace discussions. Moreover, the connections made to international norms and law, which traditionally are not the main domain of quantitative scholars, are also noteworthy. Unlike many other studies that focus only on one conflict management technique, the authors here must be commended for incorporating both mediation and arbitration. A natural next step in future studies might be to bring the process of negotiation even more into discussions of levels of peace literature. Given the stakes and audiences involved in such negotiations, how are they planned, conducted, managed, and then implemented?

As with any bold book that incorporates this many different topics, it is normal for a reader to disagree with some of the arguments in Part II. For example, rather than norm changes, the timing of decolonization might be interpreted as a matter of expediency or even as a different tool of control. In addition, the anti-secessionism norm section could discuss the implications of the support that secessionist groups receive from other countries. Nevertheless, the insights within The Puzzle of Peace create many possibilities for expansion, as future work can consider whether the authors’ framework and findings can be useful for understanding internal peace, for examining regional differences that the “zones of peace” literature would expect, and for considering the implications of the wider global (liberal) economic and political (hierarchical) order.

The main contribution of the book, particularly in terms of showing the value of adopting a multilevel, multidimensional approach to peace, cannot be overstated, as it opens additional avenues of research. Overall, this work deserves to be widely read and hopefully will lead to the engagement of many scholars from a variety of approaches with peace discussions beyond peace-as-the-absence-of-war approach.