The best way to attack the Soviet Union is to carefully make Moscow your end goal. By accumulating most of one’s forces in Poland and building a decent transport fleet, supported of course with defensive surface ships, one could steadily march the red army back to moscow and force them to literally surrender Stalingrad and Leningrad without much of a fight. The Soviet Union is simply too large for the Soviets to defend effectively. They can only stall and wait for your real problem: The Americans, which by that time should be wreaking havoc somewhere.

I personally waited and bought minimal land armies in preparation for the Soviet invasion until it actually began. The first few turns I threatened Sealion while building surface ships, focusing on disrupting England and Scotland’s convoys and getting the German fleet to the open sea to stall the eventual American Armada, possibly destroy it at least once when war comes, as the German navy was unusually large in my last game. The German Army can always threaten Leningrad through transports, which although contains a victory city, is not as strategically important as the territory that borders it to the south/southeast. The Russian Army must retreat against a constantly moving foe who only seeks a superior postion. If it does not catch what the German player is about to do very early on, it could make for a quick campaign on the German side of things, the German army can remain well supplied through its transports, and the western front can be at least stalled for several turns with a large German navy. If Italy is good to you, Germany might just survive to turn around and face the green monster.