Hey there. I attacked two cities occupied by civ x but formerly owned by civ y. Civ y had been totally conquered, but upon retaking one of their old non-capital cities I was given the opportunity to liberate it (which I did). When I came upon their original capital it only gave the options to annex or puppet... no liberate. I had to manually return the capital to them via trade at which time the game announced that civ y (china) had retaken their original capital.