<p>Much of revisionist just war theory is individualistic in nature: morality in war is just an extension of morality in interpersonal circumstances, so that killing in war is subject to the same moral principles that govern personal self-defense and defense of others. Recent work in the ethics of self-defense suggests that this individualism leads to a puzzle, which I call the puzzle of aggregation, when many threateners contribute to a single threatened harm. In this paper, I investigate the moral problems posed by the puzzle of aggregation and develop a novel account recently proposed by Jeff McMahan, which he calls “proportionality in the aggregate”. I argue that accepting proportionality in the aggregate as a moral constraint on the use of force has significant implications for the ethics of war and self-defense.</p>