If the United States turned Fascist then it would have used it's large population, exceedingly powerful industrial might, and large navy to completely wipe out the Allies, although they would suffer at first from an inexperienced and small military. read more
There’s an excellent book written on a similar scenario; check out Philip Roth’s The Plot Against America for the story of Nazi sympathizer Charles Lindbergh’s rise to power told through the eyes of a Jewish-American family. read more
If the United States had become a fascist power, then the Axis would have won. The American military and industrial might that went towards the defeat of the Axis powers would have been redirected against the British Commonwealth, with devastating effect. read more
The United States would have, under no circumstances, joined the Axis powers, so I'll just look at this as if the United States somehow stayed out of the war (which is virtually impossible). read more