Environmental issues have only recently attracted more attention in social science's reasoning about the causation of war. Even though many scholars would probably agree with the assertion that environmental degradation and demographic pressure play a role in the causal nexus of many civil wars, the question of how this relation could be conceptualized has remained open for a long time. It is the aim of Colin Kahl's book to tackle this issue not by a mere theoretical and conceptual discussion but by using two intensely studied cases as material to construct such a conceptualization and to render it empirically applicable and plausible.

Kahl's main theoretical argument is that the combination of rapid population growth, the degradation of renewable resources, and the maldistribution of renewable resources can lead to what he synthesizes as “demographic-environmental stress” (DES), which is, in his model, a root cause of civil strife. However, distinguishing his argument from a crude ecological argument about war causation, he includes the state as a switch in his model. Although he calls this approach a “state centric theory” (p. 209), it is only in weak states that the pressure ensuing from DES can lead to civil strife. Kahl distinguishes two mechanisms at work in weak states that may lead to that result: state failure, for which he takes the Philippines and Somalia as examples, or state expropriation, as could be observed in Rwanda and Kenya. According to Kahl, two intervening variables decide whether DES-induced conflicts turn violent within weak states. The first one is “groupness,” an expression that designates the fragmentation of a society into subnational groups, and the second is what Kahl calls the “inclusivity” of national institutions.

This clearly structured model is laid out in Chapter 2 of his book following Kahl's discussion of competing approaches in Chapter 1. Chapters 3 and 4 contain detailed case studies, in which Kahl meticulously constructs his argument with reference to the conflict of the National People's Army in the Philippines since the 1970s and the civil strife that arose around land issues in Kenya's Rift Valley in the 1990s. These two examples have been chosen, Kahl argues, not as test cases, but as instances of the plausibility of his argument, given the scarcity of reliable data for the two conflicts. Both case studies are based upon wide reading and indeed contain convincing statistics on demographic growth, population densities, rates of deforestation, and estimates on land shortage. In Chapter 5, Kahl discusses the role of the two intervening variables—groupness and inclusivity—for the two cases, using these to explain very convincingly the variation of violence across time and subnational regions for the two cases.

To check his model against further evidence, Chapter 6 contains a cursory look at a set of other cases. For readers familiar with these cases—Rwanda, Somalia, Chiapas, and Costa Rica—this discussion does not add much to what is already known. Furthermore, a number of competing explanations that do not stress demographic or environmental factors are not discussed at all. The real highlight of Kahl's conclusions is located in the second half, when Kahl discusses the reach of his model and its value in relation to other theories. First, he claims neo-Malthusian hypotheses about war causation to be insufficient as they do not account for the role of the state. Only in weak states, he reasserts his point, can demographic and environmental pressure be translated into civil strife (p. 237). Second, Kahl also has something interesting to say about recently advanced neoclassical arguments that highlight an abundant supply of natural resources as an incentive for waging civil war. Better known as the “greed thesis,” Kahl wants to restrict it to cases of nonrenewable resources and points out further limitations of this widely discussed concept (pp. 237–42). Approaches from political ecology are dismissed by Kahl, who deems them to be insufficiently modeled for testing.

Given his argument and the well-known estimates about both the future growth of the world's population and further environmental damage, it is hardly surprising that Kahl concludes the book on a bleak note. He predicts that young democracies, in what he still calls “the developing world,” might slip back into authoritarianism, and that there is likely to be more civil strife in those societies where high degrees of groupness make peaceful processing of internal conflicts less likely.

To my knowledge, Kahl's study is currently the most theoretically advanced attempt to include environmental degradation and demographic developments into a social science model of the causation of intrastate violence. The evidence from the cases is well presented, the argument sound and stimulating. So far, however, the model is just made plausible and not yet rigorously tested. Kahl sees this clearly and rightly points out that more rigorous testing is not yet possible given the lack of sufficiently detailed and reliable data on demographic and environmental change in states that do not belong to the Organization for Economic Cooperation and Development.

In such a situation, forms of testing would be helpful that, strangely, are not often applied in international relations, such as diachronic comparisons. Intuitively, at least a brief look into the history of the Netherlands or modern Europe in general would have led to challenging ideas. Historically, at least two factors alleviated the pressure of rapid demographic and environmental change and have proven Malthus's theory wrong: One is the enormous growth of productivity in not only agriculture but also other sectors, and the other was international migration. Strangely, neither one of these ideas appears in Kahl's reasoning about causal construction.

From a classical viewpoint of political science, there is another reason to doubt whether the popular ideas of environmental change and demographic growth are really that important as causes. Even for the two cases Kahl is dealing with, there are at least equally plausible studies that explain conflicts by using classical notions of political sociology (cf. Thomas M. McKenna, Muslim Rulers and Rebels, 1998; John T. Sidel, Capital, Coercion, and Crime, 1999, on the Philippines; and Jean-François Médard, “Le ‘Big Man’ en Afrique: Esquisse d'analyse du politicien entrepreneur,” L'Année Sociologique 42 [1992]: 167–92, for the case of Kenya). The argument has not yet been won by those who promote the role of demography and environmental degradation in the explanation of political violence. However, whoever wants to strive for that position now has an impressive and elegant study on which to build further efforts.