DescriptionI develop an account that addresses the moral permissibility of and responsibility for participating in cooperative ventures in which each member contributes marginally to a substantial, aggregate harm. I argue that a basis for assessing the morally permissibility of and responsibility for participation in such collectives should not be limited to the intentions and the causal contributions of the participant, but should also include the purpose – what I call the ‘functional role’ of the individual in the collective venture. This account has the benefit of grounding a reductionist metaphysical analysis of cooperative collective action. After developing such an account, I apply it to a particular type of cooperative, collective activity, viz., warfare. The account of contributory moral responsibility I develop provides a basis by which to ascertain moral responsibility for marginal contributions by combatants participating in an unjust war.