all here hows it pans out
us enters ww2 in sept1 1939
we fight the nazis italians & thier allies in all major campaigns
stalin fights japanese & wipes them out
hitler dies when audie murphey kills him
USA entering at that stage and Germany can forget about ...well pretty much everything.
The US fleet would have arrived by the start of 1940, the Kriegsmarine gets whiped off the map, all U-boats are lost with all hands.
when Japan gets annihilated by the Russians even more US ships would arrive.
No invasion of Norway but Norway does join the allies.
If they are smart they won't attack Denmark either, maybe set their sights on Yugoslavia.
From Norway and France the USA unleashes it airforce onto Germany.
Italy stays neutral, same for Bulgaria and Romania.
Japan becomes a Russian puppet.
By 1942, if Germany hasn't accepted the allies ultimatum of leaving Poland and Czechoslovakia and disarmament they will invade from the maginot line.
The USSR would probably attack by that time as well.
Bye bye Nazis