Can combat experience foster organizational skills that engender political collective action? We use the arbitrary assignment of troops to 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, we find that ethnically mixed districts whose veterans were exposed to greater combat exhibited greater co-ethnic immigration and minority ethnic cleansing, with minority out-migration achieved with lower loss-of-life. Further, where ethnic groups had been in complementary economic roles or the minority received greater combat experience, there was less ethnic cleansing. We interpret these results as reflecting the strategic role of ethnic cleansing and co-ethnic immigration by groups seeking political control and the role of combat experience in enhancing organizational skills at credibly threatening violence and engaging in collective action.