Quote Originally Posted by Anonymoose View Post
Perhaps the Ascians were willing to roll the dice on him [A] not turning on them and [B] causing enough chaos to bring about Zodiark's return before resistance ended. Or maybe I'm just wrong in assuming that resistance would end.
To [A], Lahabrea makes it pretty clear he never expected Thordan to betray him. He seems shocked and incredulous that he's doing so.

To [B], there's no way to know exactly. Going into the realm of theory, if the Warrior of Light died fighting the Knights of the Round and Thordan went on a "conquest of peace," doubtless the beast tribes would all summon their primals in response. If anyone fought back, there would be deaths. If it escalates to a war with Garlemald, that's an even higher death toll. All of that works in the Ascians' favor.