It's actually very, very, very simple. Well, in theory. Logistically I'm not so sure about simplicity.
But Lenin and, indeed, the entire nomenklatura of Soviet Russia revised their idea of global revolution after their failure during the 1920 Soviet-Polish War. They turned inward to build communism internally instead of seeking to spread it through military force across the globe. Simply have a Soviet victory in that war. The ideology remains that the Soviet army is necessary for spreading revolution, and they can continue with their secondary plans of invading Germany. During the 1920 war, Lenin consistently stated that following a victory over Poland, it was necessary to push on into Germany. The fact that there were large Communist groups currently active in Germany at the time, especially in the SE, helped fuel this idea. The fact that they kind of died down a bit at the same time that the Soviets lost the war helped the Party rethink their stance. But, if there is a successful Soviet occupation of Poland by 1921, they may see the decline of Communist activity as a sign that Communism is under attack and proceed to invade Germany almost immediately.
You still have rabid nationalist movements in Germany seeking to take power (Nazis, et al) but then you have the rest of Europe gearing up to fend off this Soviet steamroller. With heavy US investment in Germany, the US has a profound interest and could very well get involved at the same time.
Regardless of the reasons, this probably pulls WWII forward to the 1920's unless you have idiots like Chamberlain putting the shoe of appeasement on the Soviets instead of on Berlin.