They liked cheese
The US is concerned anytime a nation expands using methods of violence. The concern is elevated when the countries involved may cause a domino/or destabilization effect with neighboring nations.
Hoover was president from 1930 to March 4, 1933. Franklin Roosevelt then took over and served all the rest of the 30's and until 1945.
At the end of World War 2 it ceased to exist and the Nuremberg Tribunal declared it a criminal organization.
Herbert Clark Hoover, 1929-1933 Franklin Delano Roosevelt, 1933-1945