The leading preventive psychiatry recommendation for the military is keep soldiers together through training, combat and the return home. In Vietnam, soldiers often traveled there aboard a planeload of strangers as replacements doled out to units in need. Soldiers often were sent home on short notice aboard another planeload of strangers to a country divided over the war.

"That sets people up for PTSD -- for having a lot of unfinished, unresolved thoughts, feelings, emotions that they can't tell to anybody," said Col. James Stokes an Army psychiatrist.

Around the end of the Vietnam War, scientists found that the hormones boosting physical and mental strength in combat, adrenaline and cortisol also effect memory.

JAMES MCGAUGH, NEUROSCIENTIST, US IRVINE: These hormones will activate a part of your brain that's sitting there ready to be turned on. This system gets turned on; it sends instructions to the other regions of the brain where memories are being stored, and in effect it says, make a stronger memory. It's an amplifier system.

After World War II, in which a controversial study found that only 25 percent of soldiers actually fired their weapon at the enemy, the Army began using targets shaped like human beings and eventually pop-up targets to get soldiers used to the idea of hitting the real thing. By Vietnam, firing ratios approached 100 percent.

"It made a tremendous difference because now with conditioned stimulus, a man-shaped silhouette pops up in the field of view," said Lt. Col. Dave Grossman, a retired West Point psychology instructor and author of "On Killing: The Psychological Cost of Learning to Kill in War and Society."

"Conditioned response: You have a split second to engage the target. Stimulus feedback, you hit the target, the target drops. Stimulus-response, stimulus-response. What we've done is we've made killing an unthinking, conditioned reflex."