Good point. I certainly wouldn't attempt to define 'combat fatigue' too precisely; but the quote suggests that even in the Napoleonic period there was a point at which 'veterans' became stale, where more experience actually became counter-productive. I very much doubt that the evidence exists to push this much further (let alone put numbers on it), but it might be worth looking at the record of some hard fighting regiments in the last campaigns of the war. One of the reasons I thought that it didn't apply in our period was that I could see little or no sign of it in the performance of the Light Division (or the Third) in the campaigns of 1813 or 1814. And there was the crucial difference that in our period combat was relatively rare (how many days in any year did even the 95th fire their rifles?) and that it accounted for only a small proportion of soldiers' deaths - if a man decided he wasn't going to survive the war, it was because he was always getting dysentery or typhus, not being shot at!