This is a trickier case: it's not just one person switching the whole realm to another, as happened with Kabrinskia or Lurian splitoffs. Both lords switched on their own, albeit perhaps with a push (a push perhaps more by the allies than by Alaster). Maybe the lords would have switched anyways regardless of what Alaster may or may not have said... A good number of people would have.
Then, what's motivating everyone? The lords want to keep their titles, probably, and figure they have more chances to keep them in D'Hara than sticking around in Terran. Alaster probably just wants to spite Phantaria, or at least prevent the allies from using said regions against the theocracies. Pierre probably just wants to expand D'Hara. And a bunch of other actors involved in this would love to see Terran destroyed, one way or another, and peaceful (for the most part) is just easier.
This is not a case of realms joining as equals, though. Nothing will remain of Terran if this annexation goes through.
