The FBI had a strongly anti-communist direct J. Edgar Hoover and in 1947 under the FELP set up by Truman, the FBI investigated government employees to see if they were current or former members of the Communist Party. Truman gradually became more perturbed about communist spies and he therefore authorised the instigation of the Federal Employment Loyalties Programme (FELP) and the House of Un-American Activities Committee (HUAC). In September 1947, 43 witnesses were subpoenaed to appear for hearings in Washington before the HUAC, which was investigating communist subversion in Hollywood. In October 1947, eight screenwriters and two directors, known as the 'Hollywood Ten’, were imprisoned for their refusal to answer questions regarding possible communist affiliations and personal political beliefs.
The Berlin Blockade initiated on the 24th June 1948: the Soviet Union impeded access to the arteries of the three Western-held sectors of Berlin and from this Stalin endeavoured to coerce the Americans into relinquishing their sectors. However, they responded by airlifting the food and other provisions to the Western-held sectors, to which the Soviet Union did not retort, thus entailing the withdrawal of the blockade. In the same year, Alger Hiss, a former official of the US State Department, was accused of copying and passing over two hundred secret state documents to the Soviet Union. The FBI apparently encountered microfilm that contained copies of secret documents, believed to have been typed on Hiss's personal government Woodstock typewriter. Although he was not convicted of being a spy for the Soviets, he was sentenced to five years in prison for perjury.
The communist challenges influenced the U.S to wholly abandon isolationism armanent alliances; in July 1949, the North Atlantic Treaty Organisation (NATO) was ratified by the U.S and eleven other nations. However, the Soviet Union reacted with Comecon in the same year, which recuperated the planned economies of the Eastern European allies. Furthermore, two events in August 1949 increased the American fears of communism to an exceedingly great degree: firstly, Chinese communists had taken over China, headed by Mao Zedong, while the Soviets detonated their first atomic bomb – until then, U.S were the lone country which possessed the detrimental weapon.
In 1950 Julius and Ethel Rosenberg, who were Eastern European immigrants, were arrested on charges of espionage, after being convicted of selling nuclear weapons secrets to the Soviet Union during the Second World War. The onset of the trial attracted a lot of media attention and generated a largely polarised response. In June 1950, communist North Korea invaded pro-American South Korea and caused the Korean war to subsequently ensue. It was also a proxy war with the capitalist United States and United Nations allies up against the communist Soviet Union and China, which consequently brought about an upsurge in communist qualms.