This book is a very ambitious effort to account for the reason why some regions are war prone while others peaceful. The proposed explanation considers two factors, namely, what Benjamin Miller calls the “state-to-nation” ratio and the involvement of great powers in a region. The key thesis is that war is more likely when there is a mismatch between the distribution of ethnic groups and states and their boundaries. Deviations from a one-to-one correspondence between states and nations—either in the form of demands for secession or calls for national unification—provide latent sources of potential conflict between or within states in a region, although the author emphasizes how such imbalances can be overcome by nation building or power sharing.

Miller goes beyond a binary peace and war distinction and considers a series of possible outcomes, ranging from forms of “positive” or “negative” peace to “hot,” violent conflict. Great-power involvement in a region is seen as providing a dynamic factor that may account for changes between these states. Great powers can moderate the effects of regional state-to-nation imbalances (and presumably also exacerbate them, as seen recently in Georgia), but only local solutions can overcome state-to-nation imbalances and foster durable peace. The theory is evaluated through a review of patterns in conflicts since 1945, and by case studies of conflict-prone regions, such as the Middle East since World War II and the Balkans during the formation of nation-states before World War I, as well the emergence of peaceful regional relations in South America and Western Europe after World War II.

Whereas many scholars focus either on conflict between states or civil wars within states, Miller makes a good case for how the two are not always so easy to distinguish in practice, and how both interstate and intrastate conflicts may have similar underlying causes. His argument that purely structural factors, such as the relative power of actors, have limited explanatory power is also generally persuasive. Moreover, he convincingly demonstrates considerable regional variation in conflict and peace within the international system, and how nationalist aspirations and conflict over the configuration of states have been a more important source of conflict from the nineteenth century to our current era than traditional international relations theory would suggest. However, I find his claim to provide a parsimonious theory accounting for regional differences in conflict and peace somewhat overstated and not fully borne out by the book.

As I have alluded, Miller upon closer inspection actually uses the state-to-nation concept so as to encompass a great deal more than merely the fit between states and ethnic groups, in particular state consolidation and the power of central institutions. It is certainly plausible that weak states may encourage violence and that increasing state consolation may decrease challenges and violent conflict, as appears to have happened in South America. However, this seems quite different from ethnic congruence per se, and I could not find a justification for lumping all of this together as one “variable,” rather than analytically distinct phenomena. Although taking into account more features may help to better account for observed variation in conflict and peace, subsuming so many things under the state-to-nation ratio makes the concept considerably more elastic and less parsimonious than claimed at the outset.

Miller is, furthermore, not very explicit on what is meant by a “nation,” and there is a risk that state-to-nation imbalances may be noted depending on whether we see conflict or not. For example, whereas violence by Basque separatists in Spain is listed as war caused by a state-to-nation imbalance (p. 425), there is no explicit discussion of whether other communities in Spain, such as the Catalans or (more controversially) Andalucians, also contribute to state-to-nation imbalances. In addition, there is little focus on variation in organizations or mobilization. Groups are rarely unitary, and support for violent groups over moderate groups is often limited (as is the case among Basques). Moreover, there is often as much conflict between different groups claiming to represent a nation as there is with other nations/states, with Hamas and Fatah as a recent prominent example. Great-power involvement here likewise includes much more than the extent of involvement, including varieties in its form ranging from direct military assistance to regional actors to the construction of global orders (e.g., cosmopolitanism) or peace-building efforts. These may well be very important aspects, but again it seems questionable whether it is best characterized as a single variable.

The main empirical evaluation here is based on a list of conflicts, classified by causes. Miller highlights how most of the conflicts since 1945 can be said to be caused by a state-to-nation imbalance and how differences in the relative share of state-to-nation conflicts among regions fit the theory. However, looking only at conflicts disregards the distribution of the key explanatory variables among nonconflict cases. It is not controversial that many civil wars ex post can be seen to pit ethnic group against one another, but skeptics highlight how ethnic differences alone do not provide much leverage in predicting where violence will occur ex ante. A proper evaluation of the theory would require more systematic attention to nonconflict cases. South America as an emerging peaceful region does not seem an ideal case for assessing the theory since it contains few “nations” in the narrow sense. The discussion of Western Europe focuses primarily on post-1945 developments, rather than on an accounting of why state-to-nation imbalances could not be overcome in earlier periods.

Miller has written an ambitious, insightful, and engaging book, with many suggestive ideas. Although I am not persuaded that it provides a parsimonious theory of regional conflict and peace as a final product, it is extraordinarily helpful in drawing our attention to questions essential for understanding variation in conflict and peace, even if these still remain not fully understood.