In this highly accessible book, Christopher Blattman reviews and synthesises research on the causes of war, broadly defined as 'any kind of prolonged, violent struggle between groups'. Firmly in a rationalist tradition, the roots-of-war part of the book takes the bargaining model of war as the point of departure: Under normal conditions, war is never preferred to a negotiated solution, where groups agree to not carry out their threats to fight. War breaks out when the groups fail to reach this bargain. Blattman carefully reviews five categories of conditions where such bargaining failure occurs: unchecked interests, intangible incentives, uncertainty, commitment problems, and misperceptions. The paths-to-peace part of the book reviews a set of paths through which negotiated solutions become more likely, such as interdependence, checks and balances, rules and enforcement, and interventions. Throughout, the analytical argument is illustrated by highly readable anecdotes and supported by a broad literature review. In addition to combining analytical depth and accessibility, the book has many other strengths. In the admirable tradition of peace research, it draws on the relevant literature across multiple academic disciplines, demonstrating how similar approaches are in economics and parts of political science, in particular. Further uniting fields that tend to be read in isolation, the book emphasizes a fact that rationalist accounts often ignore, namely that costs and benefits of war are not always evenly distributed within groups. With 'unchecked interests', elites can go to war against the interests of group majorities, just as claimed in the democratic peace literature. Finally, Blattman reminds us throughout that war remains the exception even though violent behaviour tends to grab the headlines.