Lots of people in the US wanted to join with the CP rather than the Allies. If they do, and things go badly for France, Germany could defeat France, make Britain sue for peace, and blame everything on the French with a side order of blaming the British.
The US goes back into its isolationist hole, Germany and the CP impose harsh sanctions on France, and France goes Communist, possibly sooner rather than later. The fascists lose in Spain and communism takes hold. So the red dominos start falling in Europe, and only the U.K., Germany, Austria-Hungary, and Southern Europe including the Balkans don't go red. Japan remains fascist.
The next war is a bloodbath with the US supplying Britain and Germany against the USSR and Red Europe. The new Allies fight valiantly against the COMINTERN powers but fail to make ground for a long time, while Japan does battle with the US. The US wins by leveling Japan's cities and institutes occupation forces, shutting down communism in Asia as it can since the USSR is busy in a slog in Europe and cannot devote the necessary resources to folks like Mao Tse-Tung.
Finally, the US whips out a weapon it hadn't needed against Japan - the atomic bomb. Or rather, it spearheads the development but entrusts the use to the Allies, who drop the first bomb on Novosibirsk in Siberia. Undaunted, the Soviets fight on, thinking the bomb was a fluke. However, when a German bomber drops a more powerful bomb on St. Petersburg, Stalin gets nervous - and flees for the Urals. In his absence, the Soviets, seeing Moscow fire-bombed mercilessly, surrender, and the Germans are victorious, along with the Americans and British - somewhere around 1949.