How did the Fascist dictators' continually expanding aggression gradually erode the US commitment to neutrality and isolationism?

The short version...

How could we stand by and watch the Axis powers defeat Britain. If Britain was taken over, the Axis would gain her navy ships and the RAF.

Therefore: Lend-Lease was "all measures short of war." Also, FDR said that he didn't expect Americans to be "neutral in thought." Eventually, we cut off Japan's sources of oil and they seized the opportunity to attack us at Pearl Harbor. For all intents and purposes, we were all ready at war via military aid to Canada and Britain (and those other nations receiving Lend-Lease supplies).