Today is the 78th anniversary of the Nazi invasion of the Soviet Union. A massive sneak attack contrary to the Molotov-Ribbentrop Non-aggression pact of 1939, the invasion was delayed due to Nazi operations in the Balkans that were needed to assist Mussolini's failing invasion of Greece. The 38 day delay from the original plan of May 15th meant that Hitler's effort to invade Russia commenced exactly 129 years and 1 day after the start of Napoleon's disastrous invasion of Russia. The 38 day delay proved critical when the onset of the fall rains and then winter stopped the Wehrmacht in its tracks just 25 kilometers from Moscow.
Napoleon occupied Moscow and still failed to defeat Russia. Nazi occupation of Moscow would have been a serious setback for the Soviet Union. Moscow was the central hub of the Soviet railway network. Without it the Red Army would have been unable to move reinforcements to where they were needed. Nazi occupation of the rail hubs at Moscow would have also stopped the Soviet effort to move factories and workers further east out of reach of the Nazis.
It is possible that the Soviet Union could have survived the loss of Moscow and come back to win but we should all be thankful that theory was never tested. Over the next 3.5 years the Soviet Union defeated the Wehrmacht in the field and destroyed the Luftwaffe in the air. Had Hitler not ordered the invasion of the Soviet Union, D-Day may never have happened and would have been a much bloodier fight against a larger and more powerful force.
The turning point of World War II in Europe is the Battle of Stalingrad, fought over the winter of 1942-43. Hitler's mistake, repeating Napoleon's after 129 years and delayed by 38 days cost him victory in World War II.