A successful invasion of England would require a massive effort on Germanys part. They probably couldnt invade until 1941/2, and during that time the Red Army could build up its forces and prepare them for an assault. If Stalin attacks with 300 divisions against 100 German divisions, especially once the Red Army hasd been built up enough to handle such a task, he would steamroll all the way to Berlin before Hitler could withdraw from England. Indeed, Hitler would have to take England before the historical start of Barbarossa, as to free up troops for a strike on Russia.

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