re: If you were POTUS in December 1941

1) Japan attacked us and then officially declared war on us
2) day later Germany declared war on us

Who we sided with would stay the same. what needs to change is the power we gave the USSR. To get them on board with the UN, we gave them more votes and VETO power for life. We also gave all kinds of concessions to try to get them to join the war vs Japan. This led to the taking on Japanese territory north of japan, and Russian forces attacking Manchuria, and soviet forces occupying northern Korea (leading to the next war)

also, FDR stopping Western allied forces from entering Berlin, and dividing up the east to USSR (start of the cold war with east vs west europe)