Why do some civil wars last longer than expected, even in the face of rising costs of fighting? To explain this puzzle, Jonah Schulhofer-Wohl’s book explores quagmire as a situation that traps belligerents in fighting. Quagmire takes place if for at least one of the warring parties continuing to fight costs more than its expected benefits, but withdrawing will increase rather than avert these net costs. Using a formal model of strategic interaction between belligerent groups and foreign backers, the author explores the conditions under which, counter to established wisdom, belligerents choose to prolong fighting even as the costs of fighting rise or the stakes of the conflict decrease. Two mechanisms are responsible for quagmire: foreign assistance as a subsidy to belligerents, which reduces an armed group’s costs of fighting; and the substitution between high-cost, territorial warfare and low-cost, non-territorial fighting. In two chapters dedicated to the Lebanese Civil War, magisterially documented through archival research in multiple languages and interviews with ex-combatants, the author probes the internal validity of the theory: the logic behind the decision to continue fighting in several episodes of the war, puzzling considering the balance of power, becomes clear once the expectation of foreign backing and the costs of escalation are taken into account. The following two chapters explore the external validity of quagmire, first by analyzing the residuals of a civil war duration model, and second, by comparing and contrasting Lebanon with the civil wars in Chad and Yemen. Quagmire in Civil War is a masterpiece of conflict studies, combining a variety of research methods to explain a confusing phenomenon which tends to compound war’s dangers and bog down superpowers in foreign theaters.