Hard to say. Vietnam was an ugly war, and I don't mean that to characterize US tactics - just that all the horrors were in full display.
From my reading so far on PTSD, it seems like the brain just plain goes into self-destruct mode. Not necessarily suicide - just that it gets trapped in a situation where you can't just "let go" and "move on". Instead of how someone without might remember the trauma, someone with it seems to relive the trauma - and we humans are simply not set up to cope with being under enormous amounts of stress for prolonged periods. We need to be able to pull away from that and relax in order to function.
Without that, every snapping twig really is an enemy about to ambush you, and the only difference between "then" and "now' is that "now" is that "now" is it happening twice. Or three times. Or fifty.
That might be slightly exaggerated, but that is the impression I get. And dear Lord does it scare me.