How do you force a unit to disengage?
As in 11.3? The easiest way to do this is to cut supply to a battle hex. If there are no undisrupted units to shield them (as might be the case if the hex also has fortress or oasis supply), then they are eliminated, as they are unable to withdraw.
The less easy way is if they were already disrupted when the battle started, and you manage to eliminate all undisrupted units. At that point the disrupted units must withdraw.