Stalingrad was the turning point. It happened in the summer of 1942. The U.S. began direct military assistance to the Allies in the spring of that year.
It followed years of German expansion into the economic and agricultural core of Russia and it's sphere of influence. You take it as a given that Russia would have turned the Germans around and kept pushing forward. I believe that Hitler would have been able to more heavily fortify the East if he didn't have to worry about an impending massive invasion on the Western Front from the U.S. Most likely the Russians would have turned the Germans back into Poland/Belarus/Ukraine/Romania while the Germans re-grouped and began their renewed push, with their greater numerical and technological advantage, plus the fact that their infrastructure had not sustained a massive bombing campaign as the Soviets had.
People call the Battle of Gettysburg the turning point of the American Civil War. It happened after years of successful Southern defense in response to Northern incursion. When the South first made an incursion into the North, they were crushed and routed for the remainder of the war.
Germany was still a spring chicken in 1942, still on the offence and with the logistical boost from a claimed North Africa would have re-grouped and reorganized their expansion. (absent the involvement of the US.)
My money, absent either Soviet OR American opposition is a win for the Axis.
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