It's hard to see a path to victory for Britain and America without Germany being mauled on the Eastern front.
But, as henkster likes to say, we shouldn't assume people will act contrary to their nature. It has been Russian policy since Peter the Great to dominate Poland. So, even if Germany had not attacked Russia, the Russians would have move against them eventually.
That's a good point. The Americans and British were in a war to defeat Germany. The Russians were in a war of imperial expansion, once the first threat to the rodina was beaten off.
The Pacific was almost a private war between Japan and the United States.
There has always been a lot of speculation as to Stalin's timetable for a move west. As I generally understand it, he hoped Germany, France and Britain would get bogged down in a costly WWI style conflict which at some point the Soviets could swoop in and take what they wanted.
The quick defeat of France and relatively light casualties in the Battle of Britain (in terms of absolute manpower) threw a monkey wrench into Stalin's plans for western Europe.
I've read some opinions that Stalin intended to attack as early the summer of 1941 which explains why the Red Army did not have defensive positions established along the border because they were actually building up for an offensive when Hitler launched Operation Barbarossa.
I've read other places that he intended to attack in the 1942-1944 timeframe. I don't know. Would love to have been fluent in Russian while the Soviet archives were still open.