My charecter in Carelia was quite inactive when the war was switching from a southern war to one against the Empire, but I recall what was told to the court of Carelia was basically that CE had explained Carelia would need to change their religious views, or the war in the south would begin going poorly. Given that at that time Carelia stretched from Ser'quea to Oyonniarre, the realm would've been very very vulnerable to a Tara/CE attack. What actually happened, also regarding wether or not King Nathan ever searched for peace as was demanded, is impossible to say without the relating private IC letters.
Malic de Aquitane believes the whole war was a Caergoth/Suville/Empire plot to take down the glorious land of heroes, but it seems likely to me that some sort of meddling occurred from CE side, which Leta and company then tried to use to get Carelia into a better geopolitical position (It's troublesome being the only realm with a border with the south). After that, due to whatever reasons, the combat strength coming from the south did not match what was expected. That, and some absurd military decisions (an assault to Nida with 10 h travel time and almost even strength comes to mind..) lead to a halt in progress.
Was a fun war, I expect Carelia will be bitter to the realms around it for quite a while (the rest of its life).
