For more than a thousand years, just war theorists accepted that wars could be launched to punish or avenge wrongdoing …, the war itself being an instrument of retribution. Today, this punishment theory of just cause has completely disappeared … from international law, which recognizes only individual and collective self-defense as legitimate cause for war. But …

conflict is about rival goods, more similar groups are more likely to engage in war with each other. We formalize these ideas …