Whatever it is called, I think that national will would affect the soldiers' willingness to stand toe to toe with the enemy, and that this is something that only victory in battle or training will change. I think that it should not be a transient attribute that will improve with rest and the proximity of medical care (hospitals). It just happens that in this game the first of these is termed quality, and the second is termed disposition.
Long winded, but hopefully that made sense.

Alex