They had only just managed to stop the initial German push when the aid started coming in. Axis controlled basically all of Europe at that point and was on Moscow's doorstep, with Leningrad under siege. There's no scenario where the Soviets hold out on their own in that situation, all the Germans would have to do is consolidate and start chipping away bit by bit. Our trucks, planes and supplies are what let the Soviets keep up attacks and counter-offensives to stop them doing that.

Basically used millions of commies as cannon fodder against Germany.