Well, with the pressure from Stalin for "Second Front NOW" even from as early as 1942, I think its safe to say that without the Western Powers the Soviets wouldnt have won it, & poosibly the Soviets believed so too. The Eastern Front was a very close-run thing at more than one point.

There are as many, I would guess, who say that its all down to Hitlers insane mismanagement of it.