LMR
Back In Green!
Say you are Britain and have a mutual protection pact with France. All of a sudden, the Germans invade! When you go to help your French friends and retake their captured cities, they become your cities. Why? You have helped liberate the French, so the city should go back to them, shouldn't it? If you are in an alliance you should be able to choose whether to liberate the city, in which case it goes back to France, or take the city, where it will come under your control, much to the dismay of France, which will hurt your reputation.