If only one nation is playable in the game, the other nation is singled out as "the enemy". If you can only play Germany, Russia is your enemy in the entire game.
However, I think that's a silly distinction to make - you can't just swap nations in every game, since the game mechanics for managing a Soviet nation or army could well be different from those of managing the 3rd German Empire.
However, I think that's a silly distinction to make - you can't just swap nations in every game, since the game mechanics for managing a Soviet nation or army could well be different from those of managing the 3rd German Empire.