In my current game I am playing as a vassal of Vlandia. We are at war with Sturgia who came in and conquered Seonon. They immediately got a rebellion. Now, Seonon is controlled by Sturgian rebels, and we have no way to declare war on them. I cannot attack their parties, because they just say that they are not my enemy. It should be an option in the diplomacy screen to declare war on the rebels.