A key determinant of the success of armed groups is their ability to prevent desertion within their ranks. Whereas some attribute desertion to prospects of victory or the ability of armies to meet soldiers' basic material needs, Theodore McLauchlin develops a relational theory centering on trust and mistrust. McLauchlin argues that combatants will continue fighting when they develop norms of cooperation and mutual obligation grounded in their belief that fellow combatants share common goals and are motivated to fight for them. Combatants acquire information about each other's willingness to fight chiefly from costly actions that signal their motivations. They also extract information from social ties and subgroup memberships, but at a risk; rushes to suspect soldiers of disloyalty on the basis of stereotypes can backfire. Consequently, while threating punishment against soldiers who have attempted to desert can have a deterrent effect, threatening punishment in the absence of an actual attempt is instead likely to provoke desertion. The power of McLauchlin's argument is its ability to bridge other causally important factors – such as socialization, coercion, and ideological commitment – and show how their effects are embedded in and dependent on bonds of trust. The power of the empirical test of his argument is meticulous analysis of the Spanish Civil War, using both quantitative and qualitative data to delve into fascinating variation across military units, over time, and between Republican and rebel forces. The result of this impressive research is an important contribution to the study of civil war that encourages a shift from grand causes toward greater focus on relationships among combatants.