The upheaval in the United States after World War I (1914–1918) was due to three factors: a deadly disease, a severe economic downturn, and a fear of Communism.
About 550,000 Americans died during the influenza pandemic of 1918–19, and this number dwarfed American casualties in WWI. The movement of American troops to Europe during the war spread the disease. The disease spread panic in America because it was both lethal and mysterious.
There was a rapid economic expansion in the United States during the war. The end of the conflict caused economic tumult, however. Wages went down and labor disputes escalated as millions went on strike. In 1919, race riots exacerbated the economic unease.
The third cause of instability was the Communist threat posed by Russia. Communists seized power in Russia in 1917 and hoped to encourage Communist revolution around the globe. Also, bombs were mailed to 40 American leaders. The intense fear of Communism became known as the Red Scare, and it lasted until 1920.