Many people aren't aware that there were two Red Scare periods: one immediately after World War I, and one during the Cold War, after World War II. The first Red Scare lasted from 1919 until 1920 and the second from 1947 until 1957.
During both Red Scare periods, the United States went to great lengths to keep Communism out of the country. Federal workers were under constant scrutiny by the government to make sure they were loyal and even Hollywood celebrities were not immune to government screenings.
During the second Red Scare in particular, the U.S. was afraid that communists living in the country would work as secret spies for the Soviet Union. FBI director J. Edgar Hoover labeled Martin Luther King Jr. a communist and did everything in his power to cast doubt and discredit the civil rights leader. Communism was spreading and the public was afraid when the Soviet Union became a superpower because of the nuclear weapons it possessed.
During the investigations and subsequent trials held for suspected communists and spies, many actual spies were discovered living in the U.S. Major League Baseball's Cincinnati Reds changed its name to the Cincinnati Redlegs during the scare to make sure its name wasn't associated with Communism.
The effects of the final Red Scare ended in the mid-1950s, and although the country may have overreacted, there has never been another Red Scare in American history.