Attack Talerium for not punishing their infiltrators, attack Tara for various things they've done in the Caergoth/Suville War and for threatening to break the Federation in the first place, defend Strombran from Tara.
I mean, pick a reason, doesn't have to match reality, and run with it. But nope, all of the nobles in CE are apparently uber smart white knight clones.
Tara and Strombran have done the exact same things during the Caergoth/Suville war. Both sides continue to complain, but very little is done about it. For your reasoning, CE could just as easily attack Strombran as Tara is an older ally than Strombran. Funny thing is, the federation is weird. There is no love between Tara and Strombran at all, but still enough fear for CE so it seems.
In the meantime, the Riverholm change should normally mean that the south goes back to peace again, because Caergoth can't break their alliance with Tara, as it means suicide, how fun!
Also if your Senate blocks all initiatives, one can still secceed as I did in Sirion.
