Answer:
During the Great Depression, many Americans became disillusioned with capitalism. Others were attracted by the visible activism of the Communists over numerous social and economic causes, including the rights of African-Americans, workers and the unemployed. Others, alarmed by the rise of the Francoists in Spain and the Nazis in Germany, admired the early and firm opposition of the Soviet Union to fascism. The number of members of the Communist Party of the United States increased from 7,500 at the beginning of the decade to 55,000 for its end.
In the 1930s, the FBI began to persecute suspected communists who, according to their sources, intended to launch an armed revolt to overthrow the federal government. The Smith Act and the Alien Registration Act, laws that made illegal the opinion in favor of violently overthrowing the Government, came into force in 1940.
The entry of the United States in World War II since December of 1941 forced an alliance with the Soviet Union and thereby allowed the enrollment of communist militants in the Armed Forces. Even veterans of the Lincoln Brigade were authorized to do so and government agents proposed to use these veterans for infiltration tasks in Europe, but the Comintern required the American communists to avoid this for fear of the FBI infiltrating their ranks. On the contrary, the war allowed CPUSA militants to infiltrate new sectors of the US economy and politics, although almost always at a level of little relevance.
When the war ended in 1945 and the Cold War against the USSR began, the "truce" of the government towards the CPUSA ended and an anti-communist psychosis was aroused due to the discovery of so-called Soviet "spy networks". It began to denounce a growing power of the communists in the trade unionism of the industrial sector. Such fears were highlighted in a special way by Republican Senator Joseph McCarthy. This period is known as the "witch hunt", or Maccarthism.
