I believe the war against Russia was doomed before it even began.

While enjoying success in Crete and the Balkans, the Germans wasted a crucial few weeks to invade Russia - with winter soon after. This caught the Nazis off guard as they were initially ill-equiped for the weather, and this bought impotant time for the Soviets to prepare/regroup. I think that this seriously hindered the German advance, all the way to the end.