The main reason dath ilan would try to take Earth by force is if it looks like Earth is about to erupt an unstoppable horror that expands at lightspeed, or faster given the fundamental possibility of mages and psions, consuming all surrounding galaxies in both its own dimension and in every other resource-bearing dimension Earth can causally contact, including dath ilan, and any other worlds that can be contacted by psions, mages, and whatever psions and mages turn into after they get eaten.
Thellim either has not realized this (75%) or has not mentioned it to Isabella (25%).
[Do you know, by any chance, whether the powers of mages and psions can transmit messages faster-than-light, especially without prior slower-than-light transmission of a receiver?]