I just want to say, for all the discussion of 'could they have...' it's important to remember that Germany was never going to conquer Russia, it was a stupid (racist) idea to get Hitlers 'lebensraum' and take out Stalin's 'Jewish Bolshevist' nation (heavy on the eye-roll there). Keep in mind that Germany didn't even get Moscow, which Napoleon had actually managed to (mostly) do, and Napoleon still lost for the same reason that Germany would have regardless -- they did not have the logistical ability to support an army in an area the size of Russia. Partisan/army elements would absolutely pick apart a logistical train that long, which Germany couldn't have done any way. We have to remember Germany wasn't an actual mechanized army, it was entirely dependent on horses, and to try to use horses to haul ammunition/food/clothes/medical supplies/artillery shells/etc ~1500 kilometres from Germany to Moscow alone would be insane, especially with the millions of men and women the Soviet union had constantly attacking you.

The entire invasion was never going to work, and people give the idea it could have worked way too much credit. And this is all assuming no other nation would step in either; it's entirely on the 'nobody is in an alliance anymore' sort of fantasy world. This failed for the exact same reason that the Russian invasion of Ukraine has -- they planned for a short, easy war, because their entire ideology requires that they underestimate their foes at every available opportunity.