And both those campaigns, in terms of manpower, were nothing compared to the Russian Front. It’s almost impossible to grasp the number of people, the amount of materiel, that went into this war.
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.