In december 1941, Hitler declared war on the U.S following the japanese attack on pearl harbour.What if Hitler did not declare war on the U.S as the japanese had failed to support his invaision of the Soviet Union and even offered to send a battalion of volunteer troops to help the U.S fight the japanese?How would the U.S respond?
FDR obviously declines Hitler's aid; an alliance with the UK and ABDA in Japan is struck; it probable that FDR expands the "US Hemisphere Defense Line" into the Middle East; Backroom Deals with Churchill are likely to mean that the UK will wind up pit more squarely against Germany, while the USA might very well assume responsibility for the Pacific.
Can Germany keep FDR out of the war, perhaps even going so far as to DoW Japan to do it? Probably not--FDR will probably have his war by 1943 due to random incidents or perhaps even a propaganda exercise.
Still, if a US DoW against Germany is delayed by two years the Soviets have already won the critical campaign of 1942-3 in the East, or the worst nightmare of Germany doing so much damage to the Soviets that they are unable to fight a stand up battle against the Wehrmacht.
The first case probably plays similarly to OTL, although the Soviets probably pay a higher price in terms of numbers and their control doesn't expand as far Westward; the second case and it will take a Downfall like campaign (the use of tactical nuclear weapons) to break Germany.
Now the United States and United Kingdom are willing to do this; but this means that WW2 is going to go from a war of Tank Charges and Amphibious Assaults to WMD smashfests. This also means that things like the Holocaust continue to for an additional four years. Germany will have already solved the "Jewish Question" to its satisifaction and be well into answering the "Polish Question" as the next item on its agenda before the United States makes enough nuclear weapons to beat the Wehrmacht with them.