[nihzm]

> So killing civilians en masse is fine, as long it forces the enemy to surrender with (probably) fewer casualties

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.