For the purposes of this scenario, imagine Hitler calls off Barbarossa for one year. Because the Nazis can devote more resources against them, and without any continental allies for an even longer period of time, Britain sues for peace. Stalin realizes at this point that war is imminent and prepares for invasion. Pearl Harbor happens more or less as per OTL. Operation Barbarossa '42 is blunted by the well-prepared Soviet Army and the Nazis never reach anywhere close to Moscow.
Now what I'd really like to discuss: With that extra year of preparation, and without the costly losses of Barbarossa, the U.S.S.R. is probably capable of liberating Europe single-handedly. However, how would the Western Allies react to the idea of an entirely Red Europe? Do they launch a Sledgehammer-esque invasion of France to liberate it before the Soviets arrive? Are they likely to be too preoccupied with curb-stomping Japan to care? Would Britain act independently and re-enter the war? Would war actually be declared between the Western Allies and the Soviet Union if the Soviet Union had a realistic chance of seizing all of Nazi Germany's former conquests?