I'd just like to respond tp aog's reply.
H's right when he says it depends on the mental stability of the individual, but only to an extent.
I was never involved in the Iraq wars, (too old), but Aden, three tours of NI, and a Middle Eastern posting I can't expand on, so as I said, I know what i'm talking about.
A few months living on a knife edge, every sound, smell and visuals have to be instantly evaluated and acted upon, otherwise it could mean the death of you and/or your mates.
Then you come back home, where people, even though they sympathise, don't understand, and your still reacting, watching rooftops, avoiding certain things in case they blow up etc,
The nature of the Army does attract some people with mental problems, but it makes little difference in a combat zone, because you never know how any individual will react in any given situation, unless the are your teammates, and are tried and trusted.
It can take years,sometimes never, to adjust to civilian life, and regardless of your mental stability before you went, you are changed forever.