Despite the tragic violence we bear witness to in the world—from the Syrian civil war, to the renewed threat of genocide in South Sudan, to armed conflict in Afghanistan—war is less common today than it has been for much of human history. Scholars have provided convincing data that tracks the decline of war over the past several decades.Footnote ¹ When wars do occur, they are less deadly; fewer people die on the battlefield.Footnote ² These and related statistics are often heralded as grounds for arguing that our world has entered a new, more peaceful era.Footnote ³ The logic for such is binary: if war is declining, then peace must be increasing. But this logic has remained largely untested and unproven, until now.

The Puzzle of Peace: The Evolution of Peace in the International System takes the critical discourse on war to new theoretical and methodological grounds. The book audaciously, yet convincingly, argues that the world is, indeed, becoming more peaceful. Even to assert such a claim, the book has to conceptualize, for the first time, how to study peace as a positive phenomenon and not just as the absence of war. Through this “not-war” framework, the authors explain that “[p]eace does not just happen; it is created by the actions of states and other important political players” (p. 225). The authors then provide the first comprehensive data set of factors that give rise to peace in the international system. To do so, they trace the evolution of peace from 1900–2006 by studying data about the stability of relationships between nations on a scale of peace, ranging from security community to severe rivalry. Through this novel tracing of the evolution of peace in the international system, they establish an empirical case that “[t]he international system has become significantly more peaceful over time” (p. 70).

Written by three political scientists, all noted experts and authors on international conflict, The Puzzle of Peace breaks new ground in the study of war and peace.Footnote ⁴ In chapter 3, the authors present their argument that “[t]he world is much more peaceful in 2006 than in 1946 or 1900 when there was little or no positive peace” (p. 70). They convincingly articulate that the rise of peacefulness between states is linked to the decline of conflict over territorial issues. This is, in part, because there is less territory to fight over in the post-World War II world, and they find that “World War II constitutes the tipping point in the international system's movement toward more peace … .” (p. 8). They situate this claim amidst the work of others discussing the decline of war. They argue, for example, that Steven Pinker's basis for explaining the decline in violence in his book The Better Angels of Our Nature: Why Violence Has Declined Footnote ⁵ is “overdetermined” because he “offers far more explanations than might be necessary for his observations” (p. 74). They critique Joshua Goldstein's argument set forth in his book Winning the War on War: The Decline of Armed Conflict Worldwide Footnote ⁶ for its lack of depth by arguing that Goldstein's identification of the United Nations and its peacekeeping missions as the main factor explaining the decline of war “ignores that the organization only takes action … after the outbreak of significant violence” (p. 74, emphasis in original). They are skeptical about neorealist claims (e.g., Bradley Thayer, “Humans, Not Angels: Reasons to Doubt the Decline of War Thesis”) that attribute the decline of war to European and/or U.S. hegemony on the grounds that improvements in peaceful relations among states are not “confined to Europe” (p. 75).Footnote ⁷ Beyond these critiques of the prevailing views about the decline of war, the authors articulate important bases for why scholars should turn their investigations to peace as a positive occurrence. In doing so, the authors not only issue a call for a new discourse on peace, they also provide empirical and theoretical tools for doing so.

The book could end there. Its empirical findings would, in and of themselves, make an original and significant contribution to the field of peace studies. Instead, the authors go on to posit why the rise of peace that they observe in the international system is occurring. It is this endeavor that will likely be of the most interest to international legal scholars.

In this vein, the bulk of the book's explanatory work takes place in part II, chapters 5–9, where the historical evolution of three norms essential for understanding the rise of peace and the development of positive relationships between nations are identified and discussed. Chapter 5 examines the way that the international system has strengthened states’ normative commitments to the principle of territorial integrity since 1945. The argument is that the post-World War II international system—through its institutions and its norms—has “produced stable territorial boundaries, and the resulting stable boundaries have produced a more peaceful international system” (p. 99). The authors believe that the increase of positive peace in the international system is linked to what they call the “life cycle of territorial management norms” (p. 100), divided into three phases where the norm emerges, spreads, and becomes established and uncontested by most states. The norm against military conquest, for example, emerged after World War I but was not firmly established until after World War II, in part due to increased state commitments to enforcing violations of the norm through the U.N. Security Council and the International Court of Justice (ICJ).Footnote ⁸

Chapter 6 concerns threats to global peace arising from the creation of new states. It argues that there is a norm against secession that helps reinforce the norm of territorial integrity and stable boundaries that are so essential to maintaining peace. The legitimacy of a new state or government is connected to the peacefulness of the transfer of power. The chapter presents the empirical record for both secession and decolonization and integrates the relevant data into its theoretical findings. For example, since World War II, the success rate of secession movements has declined to 2 percent (as compared with 12 percent from 1900–1945, and 7 percent from 1816–1899).Footnote ⁹ The authors link this data to the power of the international norm against secession enforced by the need for any new state to receive substantial recognition by the international community of states. Palestine, Northern Cyprus, and South Ossetia are de facto governments that have yet to achieve statehood status due to such nonrecognition (p. 130). The norm's ability to constrain secessionist movements is even more striking when we consider another data point provided by the authors—that such moments have dramatically increased since 1945. The story of secession told by the authors provides a new perspective on an important tension in the international system—more people seek the formation of new states, while the international community as a whole remains interested in limiting the number of new states. The authors convincingly argue that the latter view is in the best interest of promoting peace overall: new states are bad for international peace.

Chapters 7 and 8 explore how the international system manages territorial conflict when it does occur. The authors articulate how the norm of uti possidetis (“as you possess”), which they define as “an international territorial norm that helps choose boundaries and is designed to promote border stability in the future” (p. 138), has worked to prevent armed conflict between nations over territory and border delimitations. They argue that prior to 1955, the norm was not yet firmly established. They then offer empirical support that states undergoing secessions followed the norm (by using prior boundary delimitations) in 59 percent of the cases as compared to 100 percent compliance with the boundary norm between 1955–2000.Footnote ¹⁰ Chapter 8 connects the operationalization of this norm to the increasing institutional capacity for pacific forms of conflict management, focusing primarily on mediation and, secondarily, on adjudication and arbitration at international courts and tribunals.Footnote ¹¹ It describes how the complexity of each and every context implicates the function of any conflict management process. “Friendly states go to courts because they trust the system as well as each other” (p. 174, emphasis in original).

Chapter 9 describes how the international legal approaches to managing maritime boundaries and sea resources play a role in establishing peaceful relations between nations. Here, the authors describe and analyze how the post-World War II maritime regime established by treaties such as the Geneva Conventions on the Law of the Sea and the United Nations Convention on the Law of the Sea (UNCLOS) have set forth new norms that promote peace. They claim that the establishment of “clear and widely accepted rules over territory can reduce state disagreements” (p. 191). Their study of the rate of maritime claims between 1900–2001 generally supports their idea: as international maritime norms became more established through the development of institutions like UNCLOS, maritime claims have been resolved primarily through diplomatic and judicial dispute resolution rather than militarized conflict.Footnote ¹² The data and analysis in both chapters 8 and 9 support the authors’ theory that the establishment of international legal institutions is good for peaceful relations among states because those regimes are more likely to resolve the disputes that arise within them.

This evidence-driven analysis provided in part II is important for international legal scholars because it shows how factors that we investigate are empirically linked to the rise and fall of peace in the international system. Norms, and the institutions that support them (e.g., affirmative cooperation through institutional frameworks, and alliances formed through regional treaties), are all empirically linked to the rise of peace. By providing data that connects international peace with international law, The Puzzle of Peace also exposes the need for more scholarly engagement by international legal scholars regarding peace.

This call for international legal scholars to consider closely empirical data from political science is not reciprocated in the book's own level of engagement with international law. Readers will not find a deep or sustained attention to international legal discourse in The Puzzle of Peace. Although the book relies on the power of international norms to explain its theory about the rise of peace, it does not engage with international legal scholarship surrounding the reason for such norms. In this way, the book's findings implicate international law without addressing it.

On the one hand, it is clearly not the authors’ intention to do so. Descriptively, they assume an international relations view, stating, for example, “from our perspective, the differences between international law, international institutions, and international norms are minor” (p. 102). The authors also state that “[a]ll approaches to international institutions, norms, and law agree that they function to regulate behavior” (p. 103). This clarity provides the reader with a straightforward account of the authors’ assumptions as aligned with their core field of expertise. The authors clearly delimit the scope of their project, albeit buried on page 101. As stated there, the aim is to explain the development of norms and their impact on state behavior, but it is “beyond the scope of this work to explain why each of the norms arose” (p. 101, emphasis in original).

Accepting this caveat, the book would have done well to engage in a more thorough study of the origins of such norms given how heavily the study relies on the work the authors believe international norms do to ensure peace in the international system. The book's central explanatory thesis rests on the power of norms in shaping peaceful state behavior and describes a set of norms that will be quite familiar to international legal scholars (although sometimes by different lexicology): the norm against territorial conquest; the norm promoting pacific resolution of international disputes and conflict; and norms against violent secessions that lead to civil wars. The book stops there when further engagement with international law would have deepened the book's impact immensely. Two such examples follow.

First, in discussing how the norm against territorial conquest promotes peace, the book references the “Grotian Moment” in international law regarding the rapid shift from World War I onward concerning norms suppressing territorial conquest, notably the norm of nonrecognition of territory acquired illegally through aggression. The book aims to trace the development of that norm through the historical arrival of legal doctrines—such as the Stimson Doctrine of 1932Footnote ¹³ and, ultimately, Article 2.4 of the UN CharterFootnote ¹⁴ —without engaging questions about why these doctrines and norms emerged (pp. 109–11). Even though the authors are clear about their avoidance of such matters, the why and the how of international norms are not so easily disconnected. When nations met to negotiate the Covenant for the League of Nations, they furthered their commitment to the principle of nonrecognition in the drafting of Article 10, in part because of a continuous practice and commitment by important states to doing so since the idea first gained ground in the late 1800s.Footnote ¹⁵ In other words, the rationale for past practices strengthened the basis for future state practice as states recognized the collective benefits of following the norm. One can trace a similar development path for the emerging international legal norm of the “responsibility to protect,” which first emerged as a doctrine in 2001, before being recognized as an international legal principle by many nations in 2005.Footnote ¹⁶ Understanding why states are more likely to subscribe to, adopt, legalize, and accept enforcement for new normative obligations seems important to the story the book tells about peace. The authors could have, for example, cited more extensive legal analysis about the historical development of the principle of nonrecognition, without having to derail their own thesis.Footnote ¹⁷

A second example of where deeper engagement with international law scholarship would have been beneficial concerns the book's treatment of uti possidetis.Footnote ¹⁸ The authors introduce the concept of uti possidetis in a scant two pages in chapter 7 before exploring its development and empirical support that nations comply with it. In other words, they do what they claim to do: explore the how but not the why. But they later conclude that conflict management and dispute resolution reinforce the principle of uti possidetis as “[g]overnments often know what is likely to happen if they agree to go to international courts, because courts such as the ICJ use uti possidetis in making their decisions” (p. 139).Footnote ¹⁹ The book relies on the International Court of Justice's decision in Qatar v. Bahrain, regarding a territorial dispute over the Hawar Islands and other land in the Persian Gulf, as support for their claims of the ICJ “mostly reaffirming the status quo (essentially uti possidetis)” (p. 177).Footnote ²⁰

Yet close attention to the Court's pleadings reveals that such a conclusion is not quite right. The ICJ did not rule on the applicability of the principle of uti possidetis in this case; instead, the Court based its determination that sovereignty over the Hawar Islands belonged to Bahrain on the grounds of an earlier 1939 British decision.Footnote ²¹ Furthermore, the Court's decision not to engage the principle of uti possidetis juris was of grave concern for some of the judges, as witnessed by the many declarations, separate opinions, and dissenting opinions in this case.Footnote ²² Such sources of information also provide further guidance about what the principle means, to both courts and states. For example, Judge Vereshchetin explains the Court's earlier understanding of the principle as stated in the 1986 decision in the Frontier Dispute case (“[I]ts primary aim [is] securing respect for the territorial boundaries at the moment when independence is achieved.”).Footnote ²³ Where territorial disputes arise from new states or new governments, the principle holds that sovereignty should be determined on the basis of continuing to possess the territory and property that the former sovereign possessed, unless otherwise provided via state consent in a treaty. Judge Kooijmans devotes three pages of his separate opinion to the meaning and applicability of uti possidetis, and reminds the Court of the previous view taken in the Frontier Dispute case that it is a “general principle logically connected with the phenomenon of obtaining independence.”Footnote ²⁴ He then explains that it applies where a noninternational boundary changes into an international boundary, not when an international boundary changes into an international boundary, as was the case in Qatar v. Bahrain.Footnote ²⁵

What does information like this do for the case that uti possidetis is a confirmed international norm? The book does not ask or answer that question, as it explicitly avoids discussion about why the norm arose. A more thorough analysis of the legal jurisprudence yields important information for understanding why the principle of uti possidetis juris has normative effect on state behavior. We can best appreciate state compliance with the practice of using preexisting boundaries to establish new ones (p. 138) when we understand why they do so. This is one way international norms become established. It is this type of legal history that helps explain why certain norms—in this case, the norm against territorial conquest and the norm for pacific resolution of disputes—have endured. A deeper engagement with international legal discourse on why norms develop would have made this book's compelling case stronger and the story the book aims to tell more impactful.

These critiques aside, readers can expect to find in The Puzzle of Peace a clear thesis and convincing evidence that our international system is becoming more peaceful over time. Many of the reasons why this seems to be happening involve factors that concern international law, such as: alliances; power; geography; institutional frameworks; and international norms. These factors matter, although precisely how they matter is a question the book frames but never aims to answer. “In short, all causal relationships are up for grabs in the study of positive peace” (p. 216). Each chapter presents its arguments followed by empirical support and concludes with a concise summary of the chapter's central conclusions. The authors integrate their three views into a text that reads seamlessly as one voice. The reader should be aware that some of the book's most essential contributions are found in the afterword, a section many might be inclined to skip. Here, it seems, the authors open up their most insightful and passionate views about the project and provide direct lessons that would be of use to policymakers and practitioners alike. Key among such insights are the following:

• • “[T]he movement to positive peace is rarely between isolated pairs of states; rather, it is usually regional” (p. 218);

• • “Although global institutions, such as the UN or the Security Council, can assist in the process of establishing positive peace from time to time, the core must remain regional cooperation” (pp. 219–20);

• • “International peace is connected to … domestic peace” (p. 221); and

• • “As one moves from a focus on war and rivalry to peace, the research agenda shifts accordingly” (p. 223).

The authors also make an important and timely critique. They take direct aim at the prevailing scholarly preoccupation with war and the failure to study peace with any depth or detail. The topic of peace was, at one time, of interest to legal scholars, particularly immediately following World War II.Footnote ²⁶ Today, peace remains a topic of interest in scholarly disciplines outside of law.Footnote ²⁷ But contemporary legal scholars have written little about peace. The Puzzle of Peace rightly asks the question: why are scholars, including those in the field of international law, so fascinated with the study of war and its absence but generally uninterested in the study of peace? For those that would resist this viewpoint, they only need look at the dearth of international legal scholarship in recent decades that directly analyzes peace and its promotion. Perhaps this is because, in part, legal scholars face the very challenge this book addresses—namely, the absence of a common conceptualization of what peace is and how it should be studied. In this way, the book provides a valuable tool for the few international legal scholars currently engaging in, and those willing to consider, the important questions about the role law should play in promoting peace in the future.Footnote ²⁸

The rich descriptive content, important methodological roadmap, original data, and apt critique make The Puzzle of Peace a new classic in the study of international peace and security. As such, it should be considered required reading for the community of international legal scholars and practitioners. It reanimates the need for law to engage with peace and provides tools for doing so. It captures the conceptual framework for understanding the importance of international norms, and with them, international law in the pursuance of international peace. Most importantly, it reanimates an essential insight familiar to past generations, but now largely forgotten: “Peace is a relationship, while war is an event” (p. 4, emphasis in original).