Forums / Questions and Answers / the 50 rule

They have to attack you for you to have any chance to attack them
You can even send your army to their city, if they attack you, they still have 50% rule, because it would not count as hostile, just defending action.
So they would need to kill one of your armies walking around (not attacking them) or if they take your city. Once they do that, you can kill them.

Other option is when they have 50% less land then you, you will have plunder and siege option I think, not take over. If they have just little bit less land then you can attack them.