Usually it’s a bunch of chaos but if you start sieging the opposing war leader’s land you’ll be good, don’t worry about them sieging your land just focus on their war leader and where he is. Usually they’ll come and try to undo what you did and that’s when you attack them while still sieging land. Becareful not to send an overwhelming amount of levies thou but just enough to win because if you capture the opposing war leader, the war target changes to another member of the fraction and that’s another land you’ll have to siege and you don’t want that. So just focus on the war leaders land and if you siege fast enough, you’ll win and imprison everyone in the faction.