In my book on tactics I say that I had seen little or no evidence of combat fatigue in our period, so I was surprised to come across this passage in the Life of Field Marshal, His Grace the Duke of Wellington edited by Sir James Edw. Alexander 2 vols (London, Henry Colburn, 1839) vol 1 p 393:
'The effect of being frequently brought into contact with death, at once wearies and appals the mind of the stoutest soldier. It is a fact, well noted by ancient no less than modern commanders, that, after long and arduous service, the veteran shrinks with disgust from the repetition of the sanguinary work, as if cloyed with destruction, - whereas the soldiers, younger in point of service, rush readily and even gaily into the battle.*'
* 'We well remember hearing a brave officer of a very distinguished British regiment observe, upon an occasion when the corps had behaved nobly and as usual sustained a heavy loss, "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." Sherer's Military Memoirs [of the Duke of Wellington]. There is an observation, also, of a similar purport in the excellent work of the historian Napier, who, in speaking of the French under Soult, and their weariness and discontent, says, that "the mind shrinks from perpetual contact with death."'
The author of this, Sir James Alexander has an entry in the DNB - he was a soldier, but too young to have served in the Peninsula or at Waterloo (born 1803 died 1885); but he had seen some service in the Russo-Turkish war in 1829 and as ADC to D'Urban in the Kaffir War in 1835, and went on to serve in Canada, the Crimea and New Zealand among other places.