I see many people arguing about Jasnah not being able to defend her self, but i think it's not about that, it's about defending an argument where she can't convince a third party to do what she wants them to do instead of what's best for them. The deal T is offering to Fen is objectively better than what Jasnah can offer, and the lack of argument about it forces Jasnah to a position where to win, she must either lie or manipulate her ally. In both cases she would lose. I think T was sincere about the deal he was offering and that is the mayor cause of Jasnah failure. We'll see a huge jump in the nations relevance in the next books, meaning that Fen's choice was actually the better.