War by Agreement presents a new theory on the ethics of war. Benbaji and Statman argue that wars can be morally justified at both the ad bellum level (the political decision to go to war) and the in bello level (the actual conduct of the military) by accepting a contractarian account of the rules governing war. According to this account, the rules of war are anchored in a mutually beneficial and fair agreement between the relevant players—the purpose of which is to promote peace and to reduce the horrors of war. This account relies on the long social contract tradition and illustrates its fruitfulness in understanding and developing the morality and the law of war.