It was characterized by the widespread belief that the United States was infiltrated by Soviet spies and communists, and that these individuals posed a threat to national security.
The hysteria was fueled by the Cold War rivalry between the United States and the Soviet Union, as well as by the Korean War and the rise of communism in China.
The anti-communist hysteria led to a number of investigations, arrests, and blacklistings of individuals who were suspected of being communists or communist sympathizers.
The most notable of these investigations was the House Un-American Activities Committee (HUAC), which held hearings to investigate alleged communist infiltration of the Hollywood film industry and other sectors of American society.
The anti-communist hysteria also had a chilling effect on free speech and intellectual freedom, as many people were afraid to express dissenting opinions for fear of being labeled as communists or communist sympathizers.
The anti-communist hysteria eventually subsided in the late 1950s, but it left a lasting legacy of fear and paranoia in the United States.