I do not think we are necessarily talking about internal war. What I am understanding is a desire for a Ruler to deal with troublesome region Lords. What I envision is an option to make the region as a whole "outlaw" to the realm. So essentially, the ruler makes that region, it's lord, and the knights serving the lord rogue. So they are no longer part of the realm. Then the realm could attack, loot, and TO the region.
Obviously, this would be a pain for the ruler. It would also make a troublesome region lord rethink his attitude, because it is now not only him but his knights as well who would be punished.
I think this would give a Ruler some decent options, with pretty severe consequences. A good balance IMO.
