"Soldiers have to learn to kill" sounds cryptic but in fact most soldiers are civilians not too interested in military warfare, but other activities. WW1 had hand to hand combat with bayonets used. Soldiers who were found dead often had complete ammo units, they had never fired a shot. I don't think this is folklore but rather real info, followed by efforts to teach soldiers about killing and then later the deprograming of killers. Where I learned it? Don't recall. Older movies are fascinating because they are not polluted or diluted by everyone's distorted perceptions of reality by what they see in movies and on TV.