This paper is concerned with the relationship between a person’s duty to perform a goal at some cost to herself and a person’s liability to be harmed in pursuit of that goal. This relationship, it is suggested, is fundamental. In order to determine whether a person is liable to be harmed as a means to a certain end, it is suggested, that person must have an enforceable duty to serve that end. This account of liability is contrasted with Jeff McMahan’s view in Killing in War. McMahan suggests that a person is liable to be harmed to avert a threat only if that person is responsible for creating the threat. By showing that there are enforceable duties that do not arise from responsibility, it is shown that this account of liability is too limited. The implications of the duty-based view of liability are then explored in the context of war, especially with respect to the harming of innocent civilians on the unjust side who have unknowingly contributed to the war-effort.