Boo to the nay-sayers who say this is not Civ as an answer. This was a cool thing that Civ did, yes. I would much prefer that either through diplomacy or conversion this was possible. I'd prefer a weak ally came to me for help and as a last resort gave up it's sovereignty as a last resort (ie they're getting crushed by another faction ) and prefer to live under my rule than the current scorched earth policy of just burning everything to the ground so you can't use it.
