Blunt the Sickle in Europe.
Italy stays out of the war.
No North or East African campaigns or occupation of French Indochina.
Japan is sold oil by the Free Dutch (who spend the money on equipping either the KNIL or any Dutch forces in Europe, if either Fortress Holland held out or any Dutch units managed to retreat south-west) as the US doesn't go for an embargo because FIC is still French.
Some US naval expansion is passed in 1940 anyway. US builds up fortresses at Midway, Johnson Atoll, Wake, Guam.
FDR manufactures casus belli, after US destroyer is fired upon by Japanese forces in the South China Sea. Enterprise, Hornet, Yorktown, and Lexington deliver massive surprise dawn raid on Truk Lagoon, crippling the air bases there and the IJN's power projection capabilities with shallow-running aerial torpedoes and dive bombing, particularly focusing on the oil storage tanks. ;-)
US Marines land on Truk, and immediately USN Construction Battalions set about creating airstrips.
Battle off Mandates. IJN and USN clash. US submarines are in position to attrite IJN, but torpedo malfunctions mean little is achieved. Truk land-based air fights through KB CAP with P-40 escorts and achieves little with level bombing B-17s and B-18s, though USAAF A-24s have some dive bombing success damaging 1 BB and 2 CA.
Really didn't mean to hit the post button at this point... oh well. Going to cut a long story short and get to a US victory in a white peace in which Japan loses its Pacific holdings and IJN most of its internal political capital, yet which leaves time for a sense of anticlimax to let the US electorate feel that this amazing victory has actually gained them nothing tangible and vote Dewey in '44...