In NBC we are starting a "campaign" in which this feature (if it were ever added) would be of great use, for now we have to "fudge" some way around it.
An example : A 120 CEH battle v 155 CEH battle, the defender considers he can hold the map for long enough to do some serious damage to the attacking force, rather than retreat without fighting, but knows there is little chance of winning, at some point in time the defender wants to retreat to safety with some of his Army, to fight another day, if the attacker has not stopped him doing this......as things stand now we have no way of doing this, UNLESS the AI "knows" what is happening and makes the Corps retreat to safety ?????.