Wouldn't have happened. A series of impressive military victories early in WWII looms large in peoples heads, but there's no getting away from the raw numbers of how outclassed Germany was wrt material and manpower against the USSR. Barbarossa was launched on very limited, low quality intelligence, and even when more accurate numbers came in regarding Soviet division numbers in 1942, the top brass refused to believe it.

Who can tell what would have happened? It sure would not have been easy without US support:

"... Lend-Lease, including 1,911 locomotives and 11,225 railcars. Trucks were also vital; by 1945, nearly a third of the trucks used by the Red Army were US-built. Trucks such as the Dodge 3⁄4-ton and Studebaker 2+1⁄2-ton were easily the best trucks available in their class on either side on the Eastern Front. American shipments of telephone cable, aluminum, canned rations and clothing were also critical."