What defeated Germany was its suicidal, unprovoked attack on the Soviet Union in 1941 in flagrant violation of the Nazi-Soviet Pact. The Reich had conquered most of Continental Europe and was importing all the raw materials in needed from Russia, which therefore it had no excuse to attack. Whatever happened, the massive invasion was doomed.
First there was the huge distance to be conquered. The late great Field Marshall Montgomery enunciated two rules of war during his retirement. Speaking in the House of Lords on May 30, 1962, he said: “Rule 1, on page 1 of the book of war, is: ‘Do not march on Moscow’...”
Even if the Germans had taken Moscow, their invasion would not have succeeded; the Soviet Government had removed east to Samara and transferred its industry to Siberia. In any event, thanks to Stalin’s Five Year plans of industrialisation, his country had more tanks and warplanes, as well as more soldiers (because of its bigger population), all fighting on their home ground. They could not lose.
Even if Japan had broken its non-aggression pact and tried to stab Russia in the back, it would have made no difference. The Japanese Army was overstretched trying to conquer China, and would have thus been able only to do what it did in 1918-22 during the anti- Bolshevik Allied Intervention in the Russian Civil War, ie merely occupy Vladivostock, leaving Siberia alone.