I was fortunate to never be shot at during all my tours in Iraq (not for lack of trying, especially on my first tour when I galavanted all over Al-Anbar on my own recognizance as a young corporal & combat correspondent; subsequent tours saw a much more subdued me), so I can't speak directly to the impact of direct fire. However I have spent plenty of time with Marines with long combat experience in "famous" battles. And there's no doubt that -- in one unit in particular -- instances of serious social disconnection & mental health issues were rampant... and untreated.
In my opinion, there can be no question that the forced sociopathy of the infantryman in early days of a campaign when every inch must be fought for, e.g. the first Ramadi, Fallujah, Baghdad excursions, is a truly "cup-filling" situation. Constant threat of death, the death and disfigurement of friends... there's no doubt in my mind that even the most mentally stout individual can have his cup filled by battle fatigue.
Thanks for all you do Mike.