i don't know, maybe it is bias against the player. we were getting crushed by both batannians, and later by the southern empire.
In my campaign the southern empire was the steamrolling faction and at the start of the war they were at 12k strength we were at 6k. Within few weeks they took half our territory, rose to 15k strength, we fell to 3-4k. we could barely keep up, struggling to recapture what we lost, trying not to loose it all. At that point i changed my tactic.
I completely stopped with the capturing and defending, ignored allied lords armies, didn't join any of their campaigns. Instead, i built a my party so that it consisted of around 60 horse archers and 40 lancers, and just started soloing around map, looking for estranged, isolated enemy lords, stalking their large armies waiting for them to disband and then strike each lord individually, capturing them and executing them. I killed their lords left and right, no discrimination, no matter the diplomatic penalty. It worked.
At the pinnacle of Southern Empire, they were at 15k strenght and had 104 lords. Now they are around 4-5k strenght and down to around 25-30 lords and lost half of their territory to us.
Now this alone does not prove 100% that we turned the tides because i killed their lords but all things considered I think its safe to say that it helped.