What were two things that caused Americans to be fearful after World War ended?
The spread of communism and the crisis
In 1945 the USA was still sunk into the Great Depression, it was quickly swept out by an unprecedented prosperity. The fear of communism fueled a tremendous fear that swiftly became the focus of American foreign policy.
America feared that the Soviet Communist would continue to push for control of all of Europe resulting in World War III. When the Soviets obtained the secrets of the Atomic Bomb, The fears of Communist aggression became fears of Thermo Nuclear War.
The Communist ideology was that Communism was an inevitable result of historical forces. The Communists told America that they would conquer the world. For a while it looked like the communists were correct. All of Eastern Europe became under the rule of communists. China's democracy felt to Communism, as did North Korea, and North Vietnam.
There were communists revolutions in Cuba, South America, and parts of Africa. Communist North Korea invaded South Korea in an effort to subjugate the South to Communism.
These fears lead to the Cold War, The Marshall plan Nato, the United States involvement in the Korean War and the Vietnam War.
Communist economics failed to keep pace with the ideology and military expansionism. The fear of Communism faded as the power of the Soviet Union broke apart.