You don't have to fight those kingdoms to the death right away; they'll accept normal peace agreements and you can go back to them when you're ready. The bars I think show the strength of those kingdoms relative to the point at which you reached that stage of the quest. Success can be triggered by the kingdoms losing to others, not just your kingdom. There isn't a quest reward as such, but those kingdoms will 'give up' on completion and disappear, rather than becoming an annoying minor clan constantly at war. You can inherit settlements when they do so, if they had any left, if a non-ruling clan chooses to join your kingdom when the old kingdom is dissolved.