I'm playing as Ottoman Empire and Aden is my vassal and ally. Every 5 years Aden declares war on Socotra, on which they have a national core. It's not difficult to make sure Aden gets control when Socotra falls, but after that Aden doesn't try to annex or vassalize Socotra. I give them a year or so to do something, then make peace for whatever money Socotra has on hand. Five years later Aden declares war again. Any suggestions on how to end this cycle?