Intra-realm wars are, IMNSHO opinion, one of the worst ideas ever proposed, and totally inappropriate for this game.
The idea that would work for this is the forced secession we used to have that, so far as I know, was only ever used once, when Alanna forced the city of Giask to secede, then went to war with it and wiped it out. 
I think it could be a good idea, or a terrible idea depending on how it was implemented. If a duke was to declare war on another, then the ruler would decide whether to ignore the action, or to support one of the sides. Perhaps there would be some sort of honor/prestige penalty for siding with the aggressor. So it would only be feasible if one duke was liked more by the ruler than the other. I know that the combat system is rather complex, so it may not even be possible. It may not have a place in BM, but I think it's an interesting concept.
