The conclusion of peace treaties following war was a norm of international politics for millennia. Since approximately 1950, however, the rate at which interstate wars have ended with a formal peace treaty has declined dramatically. I argue that the costs of concluding peace treaties have risen with the development of the modern canon of the law of war. Using an original data set, I find that states today prefer to avoid admitting to a state of war and risk placing their leaders and soldiers at risk of punishment for any violations of the law of war.