For what it's worth, how it seemed to go down IC:
Leta and her council were ambitious. They thought it a mistake to continue war in the south and threaten to expand already indefensibly lengthy frontiers - Carelia needed to consolidate. What's more, they found CE's friendship oppressive and genuinely believed that the southern realms ought to be united against a greater evil. They genuinely believed with CE's troubles up north, there was a historic opportunity to refashion the continent. Carelia had the chance to challenge or even usurp CE's strength and hegemony in Atamara by making a gambit for Skalk, Nida and - let's face it - Stargard.
There was already a lobby within Carelia that found CE influence objectionable and resented her ownership of Nida. Many did not think CE that great a friend. Meanwhile, a crisis seemed to loom. It was revealed (alleged?) that CE would support Carelia against the southern realms only if the Serpaentist lords were made to step down from their positions. The implication was made that non-compliance could/would lead to CE hostility. A fairly biased referendum asked whether to accept CE meddling in Carelian internal affairs and to abandon Carelian freedoms or to preserve those freedoms and resist hostile influences.
At some point, but a while after peace had first been made with Caergoth/Suville, the exact terms of the agreed treaty were revealed. There was some uproar about selling cities for war aid and trusting realms so easily prone to switching sides but, crucially, the affected Dukes were content in the knowledge they'd be re-situated to the duchies we acquired in war. The terms of the treaty seemed to express a confidence that Carelia would be successful in the immediate to short-term. At all times there seemed to be 100% belief that the coming war was not only just, but winnable. Likewise, that Caergoth and Suville would prove to be worthy and stalwart allies.
Anyway, my lasting impression of the whole sorry saga is that a few well placed Carelians decided a long time ago that Carelia would fight CE to the death, no matter success or failure. In the end Magna Serpaensism and free religion was a bit part to simply trying to take CE down a peg or two. It's like Carelia was always betting on an eventual northern breakthrough that would finally give her the space and opportunity to succeed against CE. You wouldn't believe the pressure that Nathan came under to conclude a peace or ceasefire last summer. I actually heard shortly before I paused that whilst he claimed CE's PM refused to answer his letters on the matter, CE's PM apparently hadn't received anything at all from Nathan. That's another thing I'd love to hear a little more about! ;-)
