There were several different events that made Americans fearful of foreigners and foreign ideas after World War I. The following are just a couple examples:
1) Russian Revolution- The overthrow of the Russian government and the newly created communist government scared Americans the most. This is because a communist system revolves around an enormous amount of government control. In a perfect communist system, the government would control how many products a business makes and how many resources a family would receive. This system allows for only a small amount of individual freedom.
2) German Revolution of 1918-1919