First off,Germany needs to take just one capital to win the game(Moscow,Great Britain,or the U.S.).I personally try to take all three capitals if I have the time or luck.This is a very difficult task to say the least.
… also they did not have an affective way to kill russian supply lines…the soviets did…
I’m not sure of the ground war (though the Germans were heavily outnumbered at the time and barely trying to hold their own), but the German U-Boat fleet really crippled Russia-British shipping for awhile in the Archangel area.
You have to hold it for one turn. However, an ally could land a fighter there on their following turn. Not that Germany has an ally, but it may apply to the Allies.