Yeah, ReiseReise nailed it. I'd add one more... send your princess to a faction you're only on mildly disagreeable terms with, and have her negotiate a marriage at a city deep inside their territory. After the wedding, your new family member will show up as a general/bodyguard stack, parked next to the settlement where she negotiated the deal. And naturally, they'll immediately declare war because you have an "army" on their territory.