How is this NOT a surrender?
It is as much of a surrender as if France during World War II merged with the UK to avoid surrendering to Germany. Maybe that's not medieval enough. But that is to say, not a surrender at all. It takes a mind-boggling amount of rules lawyering to argue you can "surrender" to a third party who is completely uninvolved in the conflict - let alone a party that is formally, if not functionally, your ally. Just because it might not be an entirely equal-sided union does not mean it is surrender by any stretch of the definition.
You would be correct in calling it a surrender if Terran made a formal plan to willingly merge with Phantaria or the Farronite Republic, because there the land would actually be ceded to the military victor responsible for inducing the merger in the first place. But for stubborn ol' Terran it's D'Hara or debellation.
