Opponent invading my land without declaring war

check your relations with him in the diplomacy tab. he may have right of way from you, and only have granted you peace

Also he may have answered yes to declare war. This often doesn't change what you see as the diplomatic state till there is a battle or a province taken.

War will not be declared until either he crosses a province center, or attacks one of your units.

Sure you get the warning about declaring war when starting to trespass, but driving on an open road won't cause war.

Stormbringer50 wrote:

check your relations with him in the diplomacy tab. he may have right of way from you, and only have granted you peace

Thanks to everyone for the answers.

The warning messages about starting war are sometimes given when you send forces to attack a declared enemy on an allies territory. Could use some work.

Related somewhat, how do I revoke "Right Of Way"?

In the diplomacy menu you can change your status with anyone. So if their status gets changed from ROW to peace, then they no longer have permission to cross your territory.

What permission they have given you won't change until they change it in that case.