Invasion without war

Have they taken a province yet? You can enter another country's territory, but as soon as you attack a unit or take a province, it will trigger war by sneak attack.

Another tactic players sometimes use is to temporarily give right of way if you haven't declared war yet. Then they can change it back after you pass the province center. It's a good way to stall for time.

Also rather than enter multiple post on the same topic across the forums, I would suggest posting once and waiting to see if it is recommended to post your issue elsewhere. I say this as I and others have posted answers to this in the other post on the exact same subject by the same OP.