In WWII Germans bombed civilian targets and allies replied in kind. Likewise, we regularly bombed civilian targets in Japan. More Japanese civilians dies from conventional bombs that the nuclear bombs.
Now if we have a bomb that accidentally kills a dozen or so civilians, it makes the front page of the newspapers (at least it did when Bush was president).
Of course, our enemy regularly kills civilians and there is little outcry from anyone.
The concept of war crimes did not exist until after the Nuremberg AND Tokyo war crime tribunals that followed the end of World War II. Yes, there was a war crime tribunal in Tokyo May 1946 - November 1948.
At the “International Military Tribunal for the Far East” (the official title), over 5,700 Japanese leaders , military and civilian, were charged with various types of war crimes.
Among the charges were POW mistreatment including execution, forced prostitution (”comfort women”) and biological warfare experimentation on prisoners including POWs.
The push for making war “more humane” has been around for about 150 years. The push for punishing war crimes has been around for half that. Both are good ideas. But good ideas can not hide the basic truth - war is about killing people and breaking things on a large scale.
Once the dogs of war have been unleashed and the cry of havoc has been yelled the options are limited to a short bloody war or a long bloody war. The Bard knew this how many centuries ago?