The fear of communism and the concern that communists were gathering information about the US in order to attack America

The question is referring to the Red Scare.

Explanation:

Red Fear was called to a time in the history of the United States in which the society and politics of the nation was suggested before the possibility that cells of communist power were implanted in the country.

Especially, the most important wave of Red Fear occurred between the late 1940s and the late 1950s, in the early years of the Cold War. During those years, there was a panic that communist spies infiltrated the government, with which began investigative campaigns aimed at "unmasking" communists within the administration. Its leader, Joseph McCarthy, founded the movement called McCarthyism from the Senate, conducting investigations against these suspects illegitimately and impartially, harming many innocent people.