Actually one time my original Aurelius Cotta gained a command for retreating. It was either outside Ambrakia or Epidamnos (not sure) but it was wooded region without actually having the trees graphic on the map. I had bad intel. Since I was outnumber I tried to intercept the reinforcement and to my surprise the initial defending stack marched up hill to help. It was going down hill from then on. I didn't want him to die as you can see he starts at the beginning as rather a very young s/c/v family member so I had him retreat as soon as all my "fight to the death" units die. His army was down to no more than enough to not get 'deleted' from campaign map.