There is a difference between 1 and 2.
1. Lords, of their own volition, joining a neighboring realm when their realm is collapsing to retain their lordship. That has happened throughout BM history and should continue.
2. Lords, under influence by their rulers, joining a neighboring realm to give a less hated realm more regions and prevent the more hated realm from conquering them. Especially when the rulers of both realms coordinated such a move with specific terms for those regions.
And hopefully without derailing this thread, I would also note that Coria-Tara seem to have undergone a similar situation as Terran-D'hara, though I don't have the same important messages to verify it, as have been reported here, or I'd have reported it. I'm interested in the verdict of this case first, as well.
