No doubt the USSR played the major role - far greater than the US (which was more of the profiteering from supplying war materiele to the combatants than actual fighting). But the concept of "National War" feeds to Russian nationalism while ignoring the contributions of smaller nations.
PS. How much of the Soviet initial losses could be attributed to Stalin's purges of the Red Army, and how much of it was a result of German tactical superiority is another issue.
wojtek