Do you suppose there might be some mileage (kilometerage...leagueage??) in clearing away any semantical fog by considering what might have been meant by the concept in the early 19th century, and what might be involved with a 20th Century version of the concept?
It is certainly interesting to get such a tantalizingly direct glimpse of the notion.
I can imagine a continuum of what might be described as "combat fatigue" that grows from what your quote describes, through the more active reluctance embodied by American troops late in the U.S. Civil War when confronted with orders to charge in manifestly foolish circumstances, through the experience of troops in the trenches of the Great War, to the relentless contact suffered by troops in various stages of WWII and other more modern conflicts. For each of the major portions of such a continuum, though, I supsect that a combatant might be unimpressed by what constituted "combat fatigue" for a combatant in the continuum's previous historical epoch!
Cheers - Howie