What's clear, looking back at history, is that Hitler would have taken Moscow before winter came if Churchill hadn't coordinated (with the US, who hadn't yet entered the war) a massive campaign to resupply Stalin because his army and air forces had been decimated by the Germans in the first weeks of Barbarossa.
American war materiel was so vital to the Soviets that the joke was, "The Red Army rides into battle in Studebaker duece-and-a-halves." They would have defended Stalingrad with nothing but spears and slingshots if not for the steady stream of arctic convoys Churchill was sending to Arkhangelsk and Murmansk.
The arctic convoys weren't steady at the time of the Stalingrad battles. Matter of fact they were suspended at that time and only resumed after the Torch invasion of North Africa in Nov 1942. Also a large portion of the aid went through Iran and Vladivostok.
My late Dad was a seaman on an American Destroyer on the North Atlantic Convoy Run. Didn’t get sunk until he was in the Med.
Remember that the USSR didn’t fight Germany for us…they fought it for themselves, and took full credit for the victory.