The moral principle that it is a crime intentionally to kill non-combatants is not any sort of revisionism, lefty or otherwise. It’s not a historical assertion or an opinion. It’s a universal moral principle.
Who, in Japan, at that time, was a non-combatant? That’s a question worth discussing.
Your view of the just use of force in the context of all out war is the leftist view. It would have resulted in far greater loss of life. The rest is second guessing.
The notion of not killing (to the point of paralysis) non combatants is a very recent concept.
When two nations are engaged in total war, there are no “non combatants” per your use. There is only “destroy the enemy’s ability, will, and support to wage war”. Every citizen is part of the war effort, even if only “consent of the governed”. One way to end war is to inflict so much damage that the populace compels the leadership to stop fighting; Japan had a pervasive culture of cultish support for the Emperor and his executing the war, and they fully supported winning WWII.
As for the Biblical angle, it is generally accepted that nations may wage war in self defense, and we have the repeated model of God Himself directing the good to END their enemy’s ability and well to wage war. I reject the notion that those OT incidents were special cases where Good allowed & directed violation of the Commandments.