I think that the Germans could not have not declare war on Russia.
Germany was anti communist and hated Russia. Even if they had not Russia would have. And if they had not the would allways be on the brink of war. And problebly if Germany had one the war and took over Britan and the USA and other Allies, Russia would still be on Germany's "HIT LIST". But Germany would have been so spread out around the world that had Russia had attack Germany they would be anielated.