Combat stress reaction
Sigmund Freud studied the effects of "war neurosis."  He believed that without too much stress, people can balance impulses (what they want to do) and prohibitions (what they know they should not do).  However, he thought that traumatic stress can cause powerful impulses that a person can no longer control.  In combat situations, he thought this could cause soldiers to run away, or to blindly attack.  He believed that soldiers would try to stop these impulses, which would lead to emotional symptoms and even loss of physical abilities.