By the way, not advocating war. In fact, could it be possible that is there were not any rules of war, we would be less willing to engage in war?
Oh, I ask in a historical sense, as in, 'what were the historical impetuses that would have caused such ethical universals between enemies?'
What about shooting down medics? Not honoring the request of surrendering troops? Execution of surrendered or captured soldiers? Why do we all follow these rules?