There is a natural (perhaps "unnatural") Immunity Factor to counter Post Traumatic Stress Disorder... It's called "sociopathy" and its evil twin "narcissism". I know, some people will only accept the opinion of Army scientists on what causes or prevents the disease, so I'll use a paraphrase of their definitions. It started to be a really really noticeable problem at the time of our Civil War and the Crimean War(s)... A time when large standing armies of conscripts were suddenly the norm rather than an aberration. A time of weapons technologies wherein somebody you could not see or hear, somebody perhaps miles away, could kill you, or your comrades, large numbers of you at a time. But the truly extraordinary stress, wasn't from the sudden spurts of violence, not directly, at least. It was the vastly longer times Between barrages, between battles... The disruption of you natural Circadian rhythms, sleep patterns and waking patterns, the "attack at dawn" because Dawn is the point at which all living creatures, from the lowest proto-bacterium to the Great Apes, and Humans, are at their lowest level of energy. So you had to be awake, and aware, by dawn. Guards have to be maintained at all hours... thus intermittent episodes of sleep deprivation. And that not on a regular basis. The randomness of that is also a factor. Then there's the acts of pure murder. You can train yourself to ignore your conscience, or be trained en masse to ignore your collective conscience... to supress the certain knowledge that you are doing something totally against your basic humanity... But the Basic Training itself, aka Indoctrination, aka Pavlovian Conditioning, aka Brainwashing, is designed not to strenghten your resolve, or your ability to think logically... but to break those barriers, those mental and spiritual bits of armor provided by God, that keep most people from committing outrageous crimes. You can, if your indoctrination is constantly reinforced, maintain that level of denial... but only for as long as your indoctrination is reinforced. Once you are separated from your army, or from your cult, or whatever group is brainwashing you, the denial becomes harder and harder to maintain. False "patriotism", or devotion to whatever OTHER similar lower-case "g" god you've been trained to worship, and a simple fear of admitting, mostly to yourself or your comrades, that you have done wrong, is a very powerful reason to keep it sublimated, to bottle it and suppress it. Alcohol and other drugs are one really commonly used method. Perhaps it's the reason members of the military are actually Encouraged to drink. Somebody who's a total drunkard might be punished or looked down upon, but so are tee-totallers. People who abstain from social drinking are actually looked upon as having antisocial tendencies, or at least, not being as suggestible, "not a team player". So, what would be a good, non-destructive vaccine, a good prevention for PTSD? not maintaining a Standing Army for one. It might sound kind of idealistic, like what would then prevent some foreign power from invading and enslaving America... Well, for one thing, we haven't been invaded since 1814.