So killing civilians en masse is fine, as long it forces the enemy to surrender with (probably) fewer casualties? Why even have laws of war then, if we adjust adjudicate these questions with a utilitarian calculus?
Of course not. Startegic calculations for warfare should not be conflated with a moral justifications for military actions. We have to come to terms with the fact that it was a morally unjustifiable decision, regardless of the effects it had on the war. This is something that too many people forget today.
> Why even have laws of war then
I think laws of war (the ones that work) are only an attempt to change the incentives that are presented to the belligerents during warfare, in such a way that the confilct is less damaging. They are not much about making the belligerents more morally virtuous in any sense other than a consequentialist / utlitarian one.
They didn't by our standards. A lot of what we think of as the laws of war today were clarified after WWII. Bombing civilians was illegal, but not in retaliation; so the US could bomb Hiroshima because the Axis had bombed Coventry. The fact that that was the Germans and probably an accident didn't matter.
If this seems extremely sketchy that's because it was, but so was Nuremberg. The Holocaust wasn't illegal for the Nazis to do to their own population - the prosecutors at the trials had to make up a standard of "behavior that shocks the conscience" that previously didn't exist in international law.
None of this reflects on morality, only legality, of course. But the legalities then were pretty primitive.
Later on of course both sides were hitting civilian targets deliberately, and using incendiaries and high explosives. But it's possible the British were the first to do it deliberately, in retaliation for the Germans doing it accidentally (which they naturally did not believe).