Two of my Uncles fought hand to hand in the Japanese held Pacific islands in WW2 -- they both were weirded out by the experience.

At least their comrades fought and died for a just cause but the post traumatic experience -- the flashbacks and in the case of Vietnam the guilt; can be more that a vet can handle.

When they say "war is hell" they are not speaking metaphorically.

When I was 15 a friend of mine in the neighborhood was an US Army Sergeant, a "grunt" in the 'nam, he was AWOL, I suppose on his way to being a deserter, he told me he just could not go back after seeing some of the guys in his platoon killed, decapitated and their heads put on poles in the ground as warning. This is what post traumatic stress is about and where the flashbacks come from. Make a long story short -- the MP's arrested him and he was sent back to fight some more.