Established accounts of the Lebanese civil war 1975-1990 fall short in accounting for its bestial incidents of ethnic cleansing. Models of irregular warfare are becoming increasingly sophisticated but do not apply to this setting of largely conventional warfare, which also defies the predictions of ethnic security dilemmas and theories focusing on the creation of homogenous ethnic homelands. To rectify this shortcoming I introduce a new explanation showing how ethnic cleansing can be used by political and military elites ferociously attacking each others’ social and military base of support in a setting where supporters can be easily identified. This new theory draws on recent work in political science that emphasizes the important role and powerful effects of social ties of support with the local population on wartime combatants including in terms of fighting capabilities. The paper subsequently tests this explanation by examining how well it explains the observed outcomes of violence against and displacement of civilians during the outbreak of civil war in Lebanon, 1975-90. This episode of the war witnessed many massacres and severe forced displacement but the observed patterns do not correlate closely with sectarian demography, imagined ethnic homelands, easily available loot, or other common explanations. I aim to show that focusing on the social foundations of prominent military rivals can explain this anomaly.