Admiting complete ignorance of the dificulties in coding, I would think that causing one army to not move would be easiest. The moving army could be chosen by CS, # of soldiers, # of TLs, or some combination of this. Or perhaps it could have something to do with home territory, ie the army that is at home stays in their region and defends (this would cause problems when both armies are leaving their realms to attack in the other realm).
If this is implemented, it would be good to allow Marshals to have two settings one for defence and the other for offense. That way they do not have to guess whether they will be successful in moving or not.
