A lot of very sensible remarks, which I agree with, if one who has never served in the armed forces nor thank God, faced battle, may offer a comment.
I think it would be useful to separate a few different states of being. I thought that Rory was referring to
(a) a condition brought on by being in too many battles too close together. I think this is quite different from simply being
(b) burnt out
(c) sick of being in the army
(d) feeling that you're getting too old/infirm for this sort of thing.
I would have thought that anyone in the armed forces and indeed in many civilian jobs would experience (b) (c) and (d) at any period in history, and feeling that at one's age a quiet life would be preferable, must be very common. But I think these are all quite different from (a), which I feel is a condition endemic to modern warfare but the Napoleonic era was probably about when it first became noticeable on a large scale. Even before that, though, I bet there were people who had seen a lot of killing and experienced a lot of personal danger at close quarters , who probably felt something like "combat fatigue".
Whatever "combat fatigue" is, the psychologists can tell us, but I'm interpreting it as a state in which too much proximity to violent death too intensively experienced ends up by making people unable to function properly. They may have a sort of nervous breakdown in which they become unable to manage at all, or be overcome by fear which previously they were able to control, or perhaps become numbed or hardened to the conditions. Either way, I'd be guessing that increasingly erratic and dysfunctional behaviour ending up (if not relieved) by either suicide, depression, flight or homicidal behaviour would be the outcome.
I think there's plenty of evidence in the memoirs I've read for all of those behaviours during the Peninsular War. And since the officers were as much in the thick of it as the men, they suffered just as much. I've seen some interesting naval memoirs which hint at the same thing. The colossal habitual alcohol consumption, including (especially?) among officers at this period also suggests what would today be called a "coping mechanism"?
Incidentally, wasn't "shell shock" really a convenient appellation for the nervous breakdown suffered by men after they'd been being shelled for a long time? Siegfried Sassoon was one who became suicidally "courageous" and had to be forcibly invalided home to save him. The fact that "shell-shock" victims like Sassoon were treated by psychiatrists suggests that the army knew that it was a psychological illness.
It would be really interesting to see a medical study of the symptoms displayed by some of the men of the Napoleonic era. Does anyone know if one has been done?