So, the mighty empire is entangled up in a civil war, you seize the day and attack the rebels; perhaps you'll be successful in carving a piece of the pie for yourself. Months go by, your wars core is ticking and all is good, then suddenly war becomes invalid as the rebels make white peace. I hope I'm not the only who considers this cheesy.
Obviously, this is a complicated issue. Ideally, the liege should inherit the war while the aggressor should be able to keep their war score, but that would create additional issues.
That being said, preventing rebel peace means that after the liege has wiped out all rebel forces, other rulers might declare war on rebels, and turn into an endless loop where the liege has to wait for the third parties give up. I think the solution would be a "pre-surrender":
Obviously, this is a complicated issue. Ideally, the liege should inherit the war while the aggressor should be able to keep their war score, but that would create additional issues.
That being said, preventing rebel peace means that after the liege has wiped out all rebel forces, other rulers might declare war on rebels, and turn into an endless loop where the liege has to wait for the third parties give up. I think the solution would be a "pre-surrender":
If the rebels want to surrender, but a third party is attacking them, all their controlled holding are transferred to their liege, and all their armies are disbanded (also, they are unable to hire mercenaries), but the war does not end until the third parties make peace. The advantage of the pre-surrender would be that no new third parties can declare war on the rebels.