While traditional research in international relations has focused more on war than on peace, greater attention has recently been paid to peace research. But we are still missing a deeper understanding of transitions from geopolitical competition to peace, especially a higher level of “warm” or “stable” peace.

Such is the objective of the new pathbreaking volume by Charles Kupchan—to provide us with a better understanding of stable peace, defined as a situation in which war has become unthinkable, and thus interstate relations are demilitarized and geopolitical competition is eliminated through peaceful and consensual change. This is a very ambitious study that is both sophisticated theoretically and rich empirically and that makes some novel and important contributions to the IR field. The amount of theoretical and empirical ground covered is indeed quite remarkable.

Two puzzles guide the book: How is a stable peace reached? And under what conditions is stable peace likely to emerge and persist? Thus, the author aims to provide a richer description and more powerful explanation of stable peace (or “zones of peace”) than what the IR literature has provided thus far. For that purpose, he is using theoretical eclecticism and integration of some of the key IR theories, both rationalist and sociological, while synthesizing major insights from realism, liberalism, and constructivism. Yet the key conceptual framework is derived from the international society approach. This is the space between international anarchy and national politics, which synthesizes elements from both.

More specifically, the analytical framework is composed of three main parts: a typology of stable peace, a description of the process by which it is reached, and an outline of the key explanatory factors. The typology is composed of three types and degrees of stable peace and zones of peace, which are examined in a number of historical cases: rapprochement (notably the United States and Great Britain between 1895 and 1906); security community (the Concert of Europe—1815–48, the European Community from 1949 to 1963, and the Association of Southeast Asian Nations from 1967 to the present); and union (the Swiss Confederation—1291 to 1848 and the Iroquois Confederation—1450 to 1777).

The sequential process to stable peace includes four stages of transition, from the international anarchy of competition to the international society of cooperation. The first stage refers to unilateral accommodation by costly concessions based on the realist considerations of strategic necessity and resource constraints. The next one includes reciprocal restraint in which both sides exchange mutual concessions as each party becomes confident that the other has benign intentions. Thus, the parties reach regularized/institutionalized cooperation best explained by liberal theories of regimes. Constructivism, for its part, best explains the final two stages of the process: It starts with a societal integration, which creates mutual trust, leading to a sense of community and “we-feeling.” This is followed by the evolution of a common identity—the parties generate and consolidate a narrative of communal identity and create social solidarity. Thus, they erode the self/other distinctions that are at the foundation of geopolitical competition.

Finally, the author specifies three causal factors that make it possible for enemies to become friends: The first is institutionalized restraint: States that practice restraint on their power at home also tend to be restrained abroad and thus to signal benign intent to potential partners, especially during the first phases of the stable peace. Whereas institutionalized restraint is a favoring but not necessary condition, the two other conditions are necessary. One is compatible social orders, under which there is much less opposition (than in incompatible social orders) to the social integration, which is a key stage in forming the stable peace. The other is cultural commonality; there are groups that have more in common (based on ethnicity, race, and religion) with some than with others, and that makes possible the formation of a shared or common identity among them—much more than with others.

Kupchan makes quite a few major contributions to the field. Space limitations allow me to mention only some of them. In contrast to the traditional/realist view of international relations as a competitive game under anarchy, he demonstrates and analyzes the theoretical as well as the empirical-historical possibility of reaching high levels of stable peace. Another major contribution is an insightful theoretical integration of the three key paradigms in IR (realism, liberalism, and constructivism) for explaining the different stages of the stable peace.

The book also provides quite a few policy-relevant insights: The study highlights the utility of diplomatic engagement rather than the type of regimes and economic interdependence for peacemaking. Thus, economic interdependence plays an important role at the onset of stable peace only when it takes place within the context of strategic restraint and political reconciliation. The study also demonstrates the utility of costly concessions and reciprocal restraint in bringing about reconciliation, as opposed to unilateral, immoderate, hard-line polices that bring about defections, harsh reactions, and escalation.

Finally, the study is empirically/historically rich: A multiplicity of fascinating and rich case studies examine the three types of stable peace. The cases are selected from different regions and historical periods and are both examples of successes to form stable peace and of failures to do so (or collapses of stable peace after it was already formed). Thus, there is a variance in outcomes that can be explained by the same analytical framework and its three key ingredients.

While this is an impressive and extremely important book, and without disagreeing with the core arguments and their examination, I wish to point out some limitations to its contribution. One is the scope of the theory application: There is tension in the book between a universal theory of peacemaking/reconciliation based on diplomacy and engagement, which should potentially apply to every case of rivalry, especially a dangerous one, and a more limited theory of stable peace, which applies only in the specific cases of social compatibility and cultural commonality. These necessary conditions for stable peace imply that the really hard cases of war and peace (such as United States–Iran; United States–China; Arab/Iran–Israeli; India–Pakistan), while benefiting quite a bit from some of Kupchan's thoughtful insights, will confront major obstacles on the way to stable peace. Thus, the theory—and the derived transforming prescription—actually applies only to the relatively easier cases of social/cultural similarity, with more limited application, if at all, to the really interesting and relevant cases in need of stable peace.

A second limitation refers to the phenomenon to be explained: the tension between peacemaking among enemies to avoid war and levels and degrees of integration. These seem to be at least partly separate questions: peacemaking among enemies who might go to war versus forms of political integration/federation/union.

Another issue is the contemporary/post–Cold War relevance of the theory. It is questionable to what extent Kupchan's theory applies to today's main type of violent conflicts, which are mostly civil wars, transborder violence, and foreign intervention related to ethno-nationalist conflicts and violent nonstate actors, notably in failed states such as those in South Asia, in the Middle East (including following the “Arab Spring”), and in parts of Africa. The key problems of stateness and ethno-national/religious identity and aspirations are completely overlooked in the analysis here. In the absence of a minimal level of stateness and national congruence, it is difficult to expect effective diplomatic engagement. Problems such as failed states and violent nonstate actors also challenge neat distinctions between national and international politics, which also locate international society in between.

One may also question whether a common identity is always a recipe for peace. While a common identity is much more than what is actually needed for peace, even a stable one, it does not always prevent wars, such as in the case of civil wars and pan-nationalist (Pan-Arab, Korea, Vietnam, China-Taiwan), and intracommunist conflicts. Common culture did not prevent nationalist wars in pre-1945 Europe and inside Islam today. Finally, hard-core realists would worry that Kupchan's idea about the pacifying effect of costly concessions can be exploited by rivals and serve as a sign of weakness.

The bottom line is, however, that despite such qualifications, this is a major, engaging, and original contribution to IR theory and to our understanding of peacemaking processes. Everyone who is interested in these subjects must read How Enemies Become Friends.