Well, historical precedent is not an end-all argument for cases like these (this isn't really a SMA case), but inasmuch as it can be useful I can't think of an historical precedent for merging two realms without war being the cause of the merger, but these cases I shown are precedents where the defeated party folded their defeated lands to an ally and not to their enemy.
The thing is, I'm sure there are plenty of times where this was tried, but the occupying army replied in a manner similar to "lol, no." and kept right on occupying. Which unfortunately is not coded into game mechanics.
