Why didn't the U. S. do anything to stop the fascist invasions of Manchuria or Ethiopia, or to stop Germany's reoccupation of the Rhineland?

Well, this answer is hugely personal, but I'd argue it's due to the US's law against international intervention (it took the Japanese attack on Pearl Harbour to finally convince the congress to join the war) and the fact that fascist ideas were pretty popular in the south of the country, namely in the regions that were sympathetic to racial segregation with whom the idea of a superior race promoted by fascist and nazi ideology was pretty popular. Again, this is based on my opinion, I am no pre-WW2-American scholar!