There are many fictional stories where Germany or Germany and Japan win World War 2. The stories usually ignore how this happens or give a few sentence reason like the Germans develop the A-bomb first.
There are other exercises in alternate history where people try to analyze the real history and try to find battles or turning point events where the Germans and Japanese win critical battles.
There is a scenario where the Germans attack without delay and effectively to prevent the evacuation of 400,000 British troops from Dunkirk. This scenario ignores that Germany logistics were stretched by the surprising speed and success of their encirclement and defeat of the main French army. The Germans had a multi-day traffic jam as they were moving across France.
France overcommitted its best troops into Belgium and the Germans went through the Ardens. There is clear evidence that in head to head battles the French did well. The French and British troops would have been able to hold a longer assault at Dunkirk.
The French and British troops holding off the Germans for the Dunkirk evacuation were dug in for urban warfare. There was no encirclement. Many later battles in WW2 lasted for months with urban warfare.
What if the Germans had turned toward the Middle East instead of the Soviet Union? What if the Germans won at Stalingrad? What if Russia lost Moscow?
The Germans were massively short of oil. The US had about 80% of the oil in the world. In Europe-Asia, Russia had about 70% of the remaining 20% of the oil. The Germans would have had to win Stalingrad and captured and kept the oil in Caucuses. This would have prolonged the war.
However, US lend-lease was providing oil, trucks and other material for the Soviets. The Soviets would still have mounted their counter-offensive. The Soviets were making good enough tanks in vastly larger numbers. The Germans winning at Stalingrad might have allowed them to avoid the Soviet encirclement. However, the Soviets would still have won one year later.
The scenarios for German or Japanese victory would have to address:
* the US had 80% of the world’s oil
* the US had higher octane fuel which enables planes that are 10% faster
* the US had a vastly larger economy and can outproduce the Germans and the Japanese combined
* the Allies had broken German and Japanese codes by 1939-1940
* the US had 27 aircraft carriers in 1945 and 70% of the large Navy ships in the world.
By war’s end in 1945, the United States Navy had added nearly 1,200 major combatant ships, including twenty-seven aircraft carriers, eight “fast” battleships, and ten prewar “old” battleships totaling over 70% of the world’s total numbers and total tonnage of naval vessels of 1,000 tons or greater. 143 aircraft carriers including conversions and the training carriers were completed prior to the end of WW2.
The US was building dozens of additional aircraft carriers in 1945. A longer war would have meant the US would have scaled up to hundreds of aircraft carriers.
The Japanese lost the battle of Midway and could not recover naval capability, while the US could lose its aircraft carriers at Midway and replace them with new construction within 12 months.
* the US had vast superiority in longer-range four-engine bombers. They had thousands at the end of the war and again the US was building a lot more. The nuclear bombs took out cities but waves of long-range conventional bombers fire-bombed Tokyo and Dresden. It would just take one or two weeks for the bomber fleets to destroy a city.
* The allies were able to put together a successful sea-lift amphibious assault on D-Day that took a couple of years of preparation, construction, planning and effort. The Germans and Japanese had nowhere near this capability. They could not assault and bring the war to the US. The Germans could not even successfully invade across the English Channel.
* Churchill and Stalin did not surrender and would not have surrendered.
* the US had the population to mobilize an army four times larger than the troops that were mobilized.