I recall something James Michener wrote about the 20th Anniversary of D-Day. It seems the French had commissioned a "commemorative" stamp which depicted the heroic French Commandos coming ashore at Normandy with a large Frog flag. Somewhere in the distance you could make out British and American flags. I know that the Russians, that is the Russians + the other captive nations that made up the former USSR, made the greatest human sacrifice of any Allied nation in WWII, but it is also true that in large measure their sacrifice was due as much to the self-delusional paranoia of their ruthless Communist dictator, as it was to German action. Whether the Russians could have/would have stood alone against Nazi Germany, as Britain did, or whether the Russians could ever have achieved victory without the material support the US provided, is definitely an open question, at least.
Well--I would prefer not to speculate whether Russia/USSR would have achieved the victory over Nazi Germany on its own without Allied assistance. This is really anything but a wild guess of what ifs. What's really important is to recognize all contributions to the victory over Nazi Germany without overlooking darker aspects of Post WWII Soviet Control of E.Europe.