What if the US policy was, for whatever butterfly causing reason you choose, is for Japan-first instead of the Germany-first agreement with the UK. How would the war in both fronts play out, would it impact the Soviet front against Germany at all, would it impact Soviet intervention at the last minute against Japan forcing it to come in earlier or not at all (seizing in OTL Manchuria, Kuril Islands, and northern Korea). Bonus points if you can butterfly a way that the USA kicks Japan's butt then turns around and says "eh, Europe, we're tired from Japan, we'll continue against NAZI Germany only enough to pre-1930 borders, then you're on your own" (possibly a result of both from Patton and Eisenhower dying in the Pacific)
Your challenge, look forward to seeing people's thoughts of how it plays out.