Many soldiers who fought in the First and Second World Wars were also traumatised; however, Vietnam was comparatively much more traumatic. Of course, other factors were also to blame (such as the chilly reception Vietnam vets received on their return), but all the evidence suggests that the biggest was how the soldiers were conditioned to kill. Three recent studies among 1,200 veterans of Vietnam, 2,797 of Iraq and 317 of the Gulf War have shown that soldiers who killed (enabled by their conditioning) are at a substantially higher risk of PTSD.

— Humankind by Rutger Bregman (Page 379)