Accidentally stole an allies capital…

(GPS5) Basically South Sudan was getting destroyed by a terrorist org and I (as the U.S.) came in and helped, but unfortunately most of South Sudans army had been wiped out so I had to reclaim Sudans territory for them. Now it says I’m the controller of their capital and other cities and I can’t give them back, and since we are allies they won’t take them back with their military. Any clue as to how I’m supposed to fix this?