Any history that CE would write would be an IC history. Revan was asking for an OOC viewpoint. The OOC fact is that, yes, CE did threaten to become involved in the southerner's war, if Carelia didn't stop the war. "End it now, or we'll step in and end it for you, and you won't like it", to paraphrase things. Carelia was politically adept enough to use that threat of meddling to turn Suville and Caergoth to her side, and start the war. Unfortunately the southerners weren't up to it militarily. (I've heard several different viewpoints on what went wrong. I have no idea which one is right. Maybe all of them?)
Now, whether that makes for valid IC justification, I have no idea. I'm not familiar with the exact details of the situation as it existed before the war started. Bedwyr could probably answer it better than I, as he was there in Carelia at the time.
