Because for that game, it seemed that 1:subs still were screwing with me 2: I had made many attempts to invade, but failed miserably every time
The key is sustainability – you have to set up a “chain” of transports that can get troops into Europe EVERY TURN. It takes a while to build that chain, but once you do, it’s the kiss of death for the Krauts.
As for subs, DDs and bombers are key. And Germany shouldn’t be buying that many subs after about turn 3 or she will run short on land units (at least, if you are taking advantage of the rule that allows you to move British and American stuff the Russia and have it converted to Russian pieces. That’s a BIG headache for Germany to overcome).