Raw recruits will often go in where veterans dare not, at least not so boldly. Veterans learn to read a battle and are more wary of the particular dangers they face, whereas inexperienced recruits are more likely to simply follow orders, unaware of the scale of the task before them. Thus have many raw divisions or regiments performed great and heroic deeds. So perhaps combat fatigue or 'battle weariness' is partly due to an increased perception in veterans of the dangers faced and the particular risks involved in an action, and thus an unwillingness to perform certain tasks. Just a thought.

Regards,
Chris