During World War II, Brigadier General S. L. A. Marshall interviewed troops that had seen action and collected data on firing rates. His results... were a shock to the American military establishment. Marshall found that among soldiers who were in combat situations, only 15-20% fired their weapons. The majority of soldiers, when it came right down to it, refused to kill; even to defend their own lives.After Marshall published his study, the military decided to revamp training so that it wasn't just teaching soldiers how to shoot, but how to kill. Human-shaped targets replaced the paper bullseye. "The firing rate among combat troops rose to 50% in Korea, and to 90% in Vietnam." But PTSD cases soared, and, in the last three decades, the military has further transformed its tactics—a greater focus on guided missiles, on bombs dropped from 30,000 feet—so as both to limit casualties and to distance soldiers from the physical act of killing; to make it, in a way, less difficult.
The non-firing majority were not cowards. They did not throw down their weapons and flee; they just refused to pull the trigger. Grossman offers data suggesting much of the Post-Traumatic Stress Disorder (PTSD) seen in veterans derives not so much from having been in danger, but from having had to kill.