Not only did the Cold War shape U.S. foreign policy, it also had a profound effect on domestic affairs. Americans had long feared radical subversion, and during the Red Scare of 1919-1920, the government had attempted to remove perceived threats to American society. Even stronger efforts were made after World War II to root out communism within the United States. Foreign events and espionage scandals contributed to the anti-communist hysteria of the period. In 1949 the Soviet Union exploded its own atomic device, which shocked Americans into believing that the United States would be the target of a Soviet attack. In 1948 Alger Hiss, who had been an assistant secretary of state and an adviser to Roosevelt at Yalta, was accused of being a communist spy by Whitaker Chambers, a former Soviet agent. Hiss denied the accusation, but in 1950 he was convicted of perjury. Finally, in 1950, the government uncovered a British-American spy network that transferred to the Soviet Union materials about the development of the atomic bomb. The capture and trial of Ethel and Julius Rosenberg for revealing atomic secrets furthered the perception of a domestic communist danger. Attorney General J. Howard McGrath declared there were many American communists, each bearing "the germ of death for society."  

When Republicans were victorious in the midterm congressional elections of 1946 and appeared ready to investigate subversive activity, the president established a Federal Employee Loyalty Program. Workers challenged about past and present associations often had little chance to fight back.  

Congress, meanwhile, embarked upon its own loyalty program. In 1947 the House Committee on Un-American Activities investigated the motion-picture industry to determine whether communist sentiments were being reflected in popular films. When some writers refused to testify, they were cited for contempt and sent to prison. In response, Hollywood capitulated and refused to hire anyone with a marginally questionable past.  

But the most vigorous anti-communist warrior was Senator Joseph R. McCarthy, a Republican from Wisconsin. He gained national attention in 1950 by claiming that he had a list of 205 known communists in the State Department. Though McCarthy subsequently changed this figure several times and failed to substantiate any of his charges, he struck a responsive public chord.  

McCarthy gained power when the Republican Party won control of the Senate in 1952. As a committee chairman, he now had a forum for his crusade. Relying on extensive press and television coverage, he continued to charge top-level officials with treachery. Playing on his tough reputation, he often used vulgarity to characterize the "vile and scurrilous" objects of his attack.  

But McCarthy went too far. Though polls showed half the public behind him, McCarthy overstepped himself by challenging the United States Army when one of his assistants was drafted. Television in its infancy brought the hearings into millions of homes. Many Americans saw McCarthy's savage tactics for the first time, and as public support began to wane, the Senate finally condemned him for his conduct.  

Until then, however, McCarthy exerted enormous power in the United States. He offered scapegoats to those worried about the stalemate in Korea or about communist gains. He heightened fears aroused by the Truman administration's own anti-communist effort and legitimized tactics that were often used against innocent people. In short, McCarthy represented the worst domestic excesses of the Cold War.