You remember the famous phrase: "no plan survives contact with the enemy".
Well, the other thing that goes away, at least for a while if you are lucky, is command and control (C2).
Last night I was playing a sandbox, fictional scenario in Scourge of War. I was in command of a Union division and I had a hell of a time pulling a brigade out of contact. It can be done, but if the morale of the troops is slightly less than super, the retreat quickly becomes a rout.