The issue with defeating the USSR is, simply put, there is just so damned much of it Straight line it is sort of impressive, being 1,000 miles from Berlin to Moscow (although Americans, especially those of us from West of the Mississippi are much more used to that sort of distance when talking travel or real world distance), but what is REALLY striking is that the A-A line is 1,600+ miles the amount of forced needed to successfully occupy that much land area, especially when you manage to turn every occupied village into a member of the "I hate the Reich Club" is simply more than Germany can manage. It is similar to my oft-repeated statement about the Japanese inj the 2nd Sino-Japanese War, way too much China and too few Japanese ( or USSR and way too few Germans).
Since you mentioned AANW, I feel comfortable in talking about the PODs, As noted already, the PODs needed to get the Germans to win are rather extreme (to the point I rather doubted if the T/L wouldn't be shredded). They required that Hitler get past his adulation of Mussolini as the First Fascist and tell him to cool his jets until the Soviets are dealt with (and equally importantly that Mussolini actually LISTENED to him), for Stalin to kill ALL of his competent Front commanders and then die of a "heart attack" enabling exactly the least qualified person, Molotov, was the one left standing when the Musical Chairs with 7.62mm Tokarev accompaniment ended,
It was extremely weak then, and hasn't really improved with age, although I didn't realize that it would wind up being th foundation for a 325 page long book.
Once you really start to look at the likely outcomes of the Great Patriotic War they range from "Stalin lives long enough to drink from Hitler's skull" to both countries bleed themselves white and both regimes wind up being toppled by revolts running from the Channel to the Caspian Sea since there are not enough secret police left to handle a bunch of farmers with agricultural tools". While there is none zero chance of the Reich winning it is vanishingly small.
Almost as unlikely as the Reich defeating the Soviets is the U.S. never engaging. Simply was not going to happen, The Japanese were bound and determined to conquer China (see above for the brilliance of that idea), and they went about it in such a ham-handed manner that they actually managed to make Americans CARE about what they were doing. Imperial Japan, for a range of strategic factors, well beyond the specific elements that, IOTL, led to Pearl Harbor was never going to really going to avoid engaging the U.S. (and most probably Britain, with both the Netherlands and France not far off the felt). U.S. goes to War with Japan, it is simply a matter of time before the Reich HAS to go to war with the U.S.
Hitler was, beyond doubt, stupid to engage the U.S. while he was hip deep in the snow outside of metro Moscow, of course the exact same thing can, and should, be said for going after his "Jewish Bolshevik" dream enemy before putting the UK on the trailer. First rule of trying to fight a two front war is to have enough personnel and enough industrial output to at least match the other side., the Reich had neither. Still, the Reich was going to HAVE to fight the U.S. at some point, simply because of the exact same sort of strategic imperatives that compelled the Japanese (and, although he didn't think of them in this way, the Reich to engage the Soviets). Minute the U.S. in a way, with anybody, the KM is in a unrecoverable death spiral. Why? Because any ship, any aircraft, any submarine not readily identifiable as American (or an ally) aka US is automatically "THEM" U-boat is stalking a convoy with U.S. merchant shipping? Could be one of THEM! "Blow that #&^@# straight to Hell". Shoot first and don't bother look at things later.
Lastly, the ability to take a large size Island, and Britain, all 81,000 square miles of it, is LARGE, almost as large as Honshu, larger than Kyushu, more than double the size of Luzon, is sometimes underestimated. The U.S. was going to assemble a fleet that would have made Overlord/Neptune seem average, even with that the Americans assumed they would take tens of thousands of casualties (some estimates topped 100K, which, given what we know today, was probably optimistic), and have to inflict perhaps a million KIA across Japan. Crossing the Channel against a Britain that had several years to recover from the material losses of 1940, had received reinforcements from across the Dominions, and likely at least a Corps sized element out of India, weapons from Canada, Australia and, yes, the United States.*
*"If you have coin, I have goods" The Raj and South Africa, not to mention Canada, could provide a lot of materials that American businesses with pay for with good U.S. dollars, with those dollars flowing straight to Ford, GM, Lockheed, Winchester, Kaiser, Fore River, etc. Capitalism is a wonderful thing, as long as you have coin or some way to get it.
