What were some aggressions and atrocities by totalitarian regimes that set the stage for the involvement of America in World War II?

The United States was paying attention to the aggression of Germany, Italy, and Japan before it became officially involved in World War II in 1941. Germany persecuted Jews through the Nuremberg Laws, and the Nazis rounded up and shot political dissenters. When Hitler annexed Austria and Czechoslovakia, some in the United States thought that war was likely, so they encouraged Congress to allow more funding for the military. Italy invaded Ethiopia and Libya.  While this did not cause much of an alarm in the United States since the invaded nations were not European, Mussolini was considered an aggressor on the same line as Adolf Hitler.
The nation that troubled the United States the most was Japan. Japan's attack on China was especially brutal. Newspaper correspondents and missionaries reported on the attacks on Nanking and Shanghai. There was also the worry that Japan could attack American islands in the Pacific. This led to the United States stationing the Pacific Fleet at Pearl Harbor in order to serve as a deterrent.