Let' say you win a battle, and play Robb Stark. The defeated player can be forced to retreat into two legal places:
a)one that would not impact him whatsoever.
b)one that if should he retreat there, it would break his supply limit and force him to destroy one of his units.
Is it possible to force him to retreat using option b?
Option 1 is the only legal option in this particular scenario so that must be selected.
What's more interesting (unlikely to come up but you can create a theoretical scenario where it could happen) is what if there are only 2 options for retreat and they both involve losing units due to lack of supply. If it's equal loss, I would say you could Robb to choose either one, because one unit is deleted and then both options become legal.
If one is more damaging than the other (lose 2 units rather than 1 to supply limits), the way the rules are phrased it appears one would become legal faster than the other and then that would apply.
A player may not retreat his defending units to an area
containing friendly units if this would cause him to
exceed his Supply limit. If a player’s only option is to
retreat to such an area, he must first destroy as many
retreating units as necessary to be compliant with his
Supply limit after retreating to the area. After taking such
losses, he may retreat the remaining units.