The turning point of the war was Operation Uranus in November '42, the Soviet counteroffensive against the combined Axis 6th Army under Paulus that were fighting in and around Stalingrad. It hit Paulus' flanks, 50 miles north and south of Stalingrad, and cut his main army off from logistical support. The goal was to force the 6th Army out of Stalingrad. The Soviets did not believe the Germans would stay, once surrounded.
When other German forces were redirected south to try to reopen the logistical lanes, instead of allowing the 6th Army to fight their way west to meet up with the relief forces, Hitler ordered them to stay put in and around Stalingrad, and not give up any ground.
The relief forces were unable to break through, Paulus' army was besieged, and went down to defeat, needlessly costing the Germans 250,000 men.
None of that would have been possible had the USA not been supplying the Red Army with everything from airplanes and bombs to tents and boots.
