They reacted rather poorly. They were afraid of another war and wanted to prevent it at all costs, so they didn't bother fascists like Mussolini or Nazism led by Hitler. They supported the policies of appeasement, which meant that they were prepared to give them what they wanted if that meant avoiding more wars since they were still recovering after world war 1.
The Vietnam War had a profound effect on America. The war also drastically decreased Americans' trust in political leaders. In foreign policy, the U.S. suffered from the so-called Vietnam Syndrome: a fear of getting involved in foreign ground wars that might become long, bloody stalemates with no foreseeable end.
