In the previous chapter, I argued that the rules of war can be supported by the largely consensual ideas of honor and collective responsibility. In this chapter, I will explain how some of these rules of war can also be supported by minimal natural law considerations, especially those having to do with the protection of the vulnerable. The general idea that there should be restraints on the behavior of soldiers during war was supported in the previous chapter. Now I begin the task of explaining why certain behaviors of soldiers in nearly every wartime situation have been and should be proscribed by the rules of war. Perhaps surprisingly, this discussion does not lead me into considerations of justice, although there is surely a place for justice in the rules of war. Instead, I will be investigating the role of humaneness or mercy in the rules of war, an idea that we will see in Chapter 4 is the cornerstone of the main international legal documents that treat the rules of war.

The idea that there is a jus gentium, a law of nations or a law of peoples, is longstanding. Some have argued that such a law can only be understood as a set of agreements among nations that shift over time, but that may have a common core of consensus. Indeed, contemporary courts have equated the law of nations with customary international law. In the previous chapter, I suggested that collective responsibility might ground such a consensus.