A cheap tactic that should be removed

So a couple weeks ago I was playing a match when I noticed a country that was a threat to mine had left their capital empty. Knowing that I couple occupy empty cities if I was his ally, I decided to ally him and take over his capital the next turn (dick move I know). However, what I didn't expect is that since it took 2 turns for him to un-ally me and the declare war on me, he instantly lost because I technically held his capital for 2 turns. What I thought was a good way to gain an early advantage in a war was really the most overpowered tactic in the game.
Can this be fixed so that it's only after 2 turns while in war and not 2 turns in general?

Or they can make the guy getting his cities allycap have to accpet it, kinda like he has to accept peace and allies?

this way you kill two birds with one stone, no more allycaping and no more killing people with it.

Not bug, it happen in real life as well> Traitor are everwhere.

One tip buddy: never ally. If you really wanna ally, make sure none of your cities are empty.