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The game (I would say rightly) prevents us from declaring war on vassals of other factions. You have to attack the parent faction and all their vassals. However, I have a NAP with Gongsun Zan and he's declaring war on my (biggest and most profitable) vassal, Yuan Shu. My only options are to abandon my vassal or gain treachery for breaking the NAP, even though its really the AI that is violating the NAP.