We create the conditions for the next war usually by trying to avoid the last war.
And, this has nothing to do with laws. Laws are created by people, not the gods.
Revenge is the issue. And, it is perfectly natural. To be civilized we need to control it.
It is like controlling the ingress of foreigners from “over the next hill”. When they contact us we get sick (flu, etc). So, they must be dirty and evil. To keep them from expanding in our direction we must kill them at every opportunity — for out own health and survival.
Perfectly natural, and in accordance with evolution. Being civilized means we must control this.