Once the Axis powers had been defeated during WWII, the shaky relationship between the Soviet Union and the United States began to disintegrate. This marked the beginning of what we would know as The Cold War. When the US was concerned about the threat of a communist takeover, causing our leaders to make attempts, both internationally and domestically, that would contain the communist threat.
Historians frequently debate what caused the Cold War. Was it the threat of Soviet expansion, or was it a reaction to America's foreign policy? After the detonation of the atomic bomb in Japan, during WWII, tensions increased among the two super powers. As the US flexed it's muscle, the Soviet scrambled to develop the same type of military, particularly nuclear, capabilities. Each nation demanded that the other give up development and possession of further nuclear capabilities, but distrust caused them both to refuse. In March 1947, the President put forth a document known as The Truman Doctrine. This document officially declared communism a threat to not only the US but to all peoples of the world. It was now the responsibility of the US to defend the globe from totalitarian regimes. This proclamation instilled fear in the hearts of many Americans, and convinced Congress to grant Truman the funding to aid Greece and Turkey in establishing democratic governments.
This containment theory declared that the US needed to remain vigilant, and not allow for Russia to gain influence over weaker nations. This constant paranoia abroad began to take effect internally. Shortly after he released the Truman doctrine, he declared that all employees of the federal government were subject to investigation in order to determine their loyalty. The US government was convinced that the Soviet Union had American spies who had infiltrated all layers of government, and were releasing confidential information to them. In 1933 US official intercepted a Soviet message that revealed financing for an American Communist Party, and then in 1944 information was received that they were in fact on the road to developing nuclear capabilities.
In 1947 US Congress passed the National Security Act which led to the creation of agencies such as the National Security Council (NSC) and the Central Intelligence Agency (CIA). These agencies were responsible for conducting covert activities and information gathering, which would influence the execution of US foreign policy. In addition to military strategy, the US also developed and economic strategy that would expand it's influence. After WWII, Europe was left destroyed and in economic shambles. Therefor in 1947 the Marshall Plan was declared; it would allow the US to provide economic aid these nations in need.The post-WWII American military lost a lot of men and resources during the war, and Truman was determined to strengthen our forces back up. Then in 1948 Soviet and American conflict began to increase in Germany. As US policy makers and our allies began to mobilize, in an attempt to integrate the separate areas of the Federal Republic of Germany, the Soviet moved even quicker, as they blocked off all roads connecting to Berlin. This created another opportunity for the US to show its muscle, as they flew over the blockade, and distributed aid to Berliners.
During the 1948 election, the majority of Americans were convinced of the Communist threat, which allowed Truman to gain popularity, and rally support. His hard-lined policy of communist containment, would soon lead the US into another devastating war, which was simply an extension of the Cold War, the Korean War. In June of 1950, the communist regime of North Korea invaded South Korea. North Korea was back by the Soviet Union, which caused the United States to view this act of aggression as the Soviet's attempt to expand it's influence. However the situation was extremely complex, with China at North Korea's border, the US had to be cautious in not instigating involvement of Mao Zedong, and his regime over The People's Republic of China. However when General Douglas MacArthur boldly led his troops closer to the border, causing China to send in reinforcements. Disagreements as to what to do next emerged between the more aggressive MacArthur, and President Truman. Eventually Truman ordered MacArthur to return home in order to avoid further escalation.
As the tensions in Korea died down, the US began to reach out to countries like Japan and Latin America, in order to further contain the communist movement. Additionally, the US government began to launch media campaigns to instill anti-communist feelings among Americans and the people of the world. This fueled baseless communist accusations and witch hunts all over the country. Union memberships suffered greatly, as people began to associate them with the communist movement. Lists were created in certain industries, which contained names of alleged communists, and prevented them from obtaining employment and other opportunities. The US federal government denied the existence of such lists, but none the less, there was an increasing sense of unease and distrust even among fellow Americans.