This article examines the impact of military unit composition on desertion in civil wars. I argue that military units face an increased risk of desertion if they cannot develop norms of cooperation. This is a challenging task in the context of divided and ambiguous individual loyalties found in civil wars. Norms of cooperation emerge, above all, from soldiers sending each other costly signals of their commitment. Social and factional ties also shape these norms, albeit in a more limited fashion. Hence, unit composition can serve as an intervening variable explaining how collective aims can sometimes induce individual soldiers to keep fighting. Analyzing original data from the Spanish Civil War (1936 - 1939), I demonstrate that three characteristics of a military unit's composition - the presence of conscripts rather than volunteers, social heterogeneity (whose effect is found to be limited to volunteer units), and polarization among factions - increase the individual soldier's propensity to desert. Unit composition proves at least as important as individual characteristics when explaining desertion. This analysis indicates the usefulness of moving beyond commonly used atomistic understandings of combatant behavior. Instead, it suggests the importance of theoretical microfoundations that emphasize norms of cooperation among groups of combatants.