“We are reluctant to admit that essentially war is the business of killing,” Marshall wrote, while the soldier himself “comes from a civilization in which aggression, connected with the taking of life, is prohibited and unacceptable.” The Army, having just fought the Second World War, embraced Marshall’s findings.

Within months, Army units were receiving a “Revised Program of Instruction,” which instituted many of Marshall’s doctrines. It was no longer sufficient to teach a man to shoot a target; the Army must also condition him to kill, and the way to do it, paradoxically, was to play down the fact that shooting equals killing. “We need to free the rifleman’s mind with respect to the nature of targets,” Marshall wrote. A soldier who has learned to squeeze off careful rounds at a target will take the time, in combat, to consider the humanity of the man he is about to shoot. Along with conventional marksmanship, soldiers now acquired the skill of “massing fire” against riverbanks, trees, hillcrests, and other places where enemy soldiers might lurk. “The average firer will have less resistance to firing on a house or tree than upon a human being,” Marshall added. Once the Army put his notions into practice, they bore spectacular results. By the time of the Vietnam War, according to internal Army estimates, as many as ninety per cent of soldiers were shooting back. And some were paying a price.