Units running around in my country?

[Omghax]

Hi. Are you both at war with the same nation? That will also allow them to enter your territory.

 

[Omghax]

That'd likely be why anyways. Being at war with the same nation will give you both access through each others land.

 

[Omghax]

To me the game will come to a full stop late game, because soviet units will move into american territory (after their invasion in europe) and american units move into soviet territory and none will be able to attack anymore (In the wiki it says that they can, but in my 3 games so far they never could).

Do you have a save where this has happened?