Can combat experience foster organizational skills that engender political collective action? We use the arbitrary assignment of troops to frontline combat in World War II to identify the effect of combat experience on two channels that change local ethnic composition and future political control: ethnic cleansing and co-ethnic immigration. During the Partition of South Asia in 1947, an environment where national borders were themselves endogenous to ethnic composition, we find that ethnically mixed districts whose veterans gained more combat experience exhibit greater co-ethnic immigration and ethnic cleansing. However, where ethnic groups had been in complementary economic roles or the minority received greater combat experience, there was relatively less minority ethnic cleansing. We interpret these results as reflecting the substitute roles of ethnic cleansing and co-ethnic immigration in altering local ethnic composition to gain political control and the role of combat experience in enhancing organizational skill that facilitates political collective action.