The “law” of war deals with wars between nations.

Wars between nations used to be the norm. Many people considered warfare to be fine and noble, the highest type of human endeavor. "Is not peace an element of civil corruption and war a purification, a liberation, an enormous hope?" (Thomas Mann)

Those ideas wore themselves out in the trenches of the First World War.

The nations of the world, including the United States, renounced war as an instrument...