I'd say thats a very good idea, but as you say, if your city is undefended and the ai has a big enough army, it wouldn't care if you saved it entire nation from utter annihalation, gave them all the jewelery that makes up their crown jewels and gave them back every single territory they lost, they would still stab you in the back for one piece of wortheless land and then realise that they will lose everything else.