Medical ethics - not medical techniques - are probably the leading way to decrease or increase deaths due to war

How doctors and nurses morally regard all of their fellow human beings, rather than how they medically treat their actual, relatively few, patients, is probably the number one determinate in whether wars are relatively bloodless or particularly bloody.

The entire culture takes many of its moral cues from the medical professionals and when they (as in WWII Germany and America )  sanction or even advocate neglecting or killing those judged lesser than others, this attitude bleeds across the whole country and into the actions of its troops --with horrendous consequences.