Something that cannot be emphasized enough is the war weariness that the American people had after the Second American Civil War from 1935-37. The US was no “sleeping giant,” as it might have been without it and to some degree was in WWI. No, it was a bloodied and demoralized nation with a lot of hostile Latin American nations that also required focus and energy.
The country was willing to go to war to stop Germany and Japan after the Month of Provocation, but only for limited goals such as open sea lanes and the Pacific possessions. (The Philippines gained independence in 1936 here, and swiftly moved into the Japanese orbit).