Indeed, the study of killing by military scientists, historians, and psychologists gives us good reason to feel optimistic about human nature, for it reveals that almost all of us are overwhelmingly reluctant to kill a member of our own species, under just about any circumstance. Yet this understanding has also propelled armies to develop sophisticated methods for overcoming our innate aversion to killing, and, as a result, we have seen a sharp increase in the magnitude and frequency of post-traumatic response among combat veterans. Because human beings are astonishingly resilient, most soldiers who return from war will be fine. But some will need help coping with memories of violence.