The Soviet Union could not have won without US support of arms and materiel. It might have been able to survive in some form by completely abandoning everything east of the Urals. US industrial production was crucial to Soviet survival, not a nice to have. Costly business, dividing up Eastern Europe in the Molotov-Ribbentrop Pact.
Russia stopped the Germans by building more than 34000 tanks and sacrificing millions of lives. The US only really entered the war after 4 years of sitting on the side lines. Yes US support of materials to Europe was important but don’t overplay it’s significance. Also, remember that Europe paid for the US materials in gold. The US was basically acting as an arms dealer, with the 2nd world war being a huge business opportunity for US manufacturing, and the direct reason for the US moving ahead of Europe.
But it had beaten back most if the invasion before US materiel showed up in substantial quantity, and its industrial output (also) grew dramatically throughout the war.
Whether it could have secured enough food to sustain that output, without US SPAM, is unclear; the Soviets openly acknowledged the importance of the SPAM. The hundreds of thousands of trucks, millions of tires, millions of tons of steel, and corresponding amount of refined petrolem the US delivered probably mattered too, when it finally got there. Soviet pilots liked the P-39s, sort of proto-A10s.
The Soviet materiel placement in 1941, immediately before Germany invaded, was consistent with immediate plans to invade Germany.
