I think Howie might have something there. What exactly do you mean by combat fatigue? is it a general condition brought on by long exposures to a particular combat environment? A particular set of behaviors? If you can describe the behaviors and 'symptoms' for it, I imagine you will find descriptions of it by behavior and symptom.
"it is almost time our old hands should be sent home; they have had too much of this; they were as steady as usual but not in such good humour as the men who last came out, - a few more such victories would sicken them."
This is pretty vague, even if it is suggestive. Is combat fatigue not being in 'good humour' and possibly sick of more victories? Any veteran might be in less than good humor and sick of the whole thing after years of campaigning without necessarily suffering 'combat fatigue.' When I think of combat fatigue, I think of the 'thousand yard stare' described by veterans of the Pacific campaigns in WWII, or a game term for troops that have been on the firing line too long.
So, what do you see as the behaviors and symptoms of combat fatigue?