They’d probably still have sold weapons to Hitler if the Japanese weren’t allied with Germany and it was just a US-Japan war.

"Catch-22" jokes about how American weapons manufactures did in fact continue to provide military aid to both sides of the war even after we'd officially joined on the side of the Allies.

But I think a lot of that was rooted in the fact that FDR inherited a fascist government from Hoover. And it was very difficult to get the anti-Communists to keep sending kickbacks to their favorite White Terror factions all through Europe and East Asia. Hell, its a bit of a miracle that Churchill and FDR were able to align with Stalin or Mao at all. Under different political conditions, it could easily have been a three-way conflict that dragged on straight through the fall of Berlin.

Certainly, more than a few American generals were convinced they should have started dropping bombs straight into Moscow and Beijing. The only thing that really held them up was the exhausted state of the US economy by the time Tokyo surrendered.