The Truman Doctrine, announced in 1947, was a speech given by President Truman asking for U.S. assistance for Turkey and Greece in order to prevent the spread of Communism. It is often seen as the beginning of the Cold War.
After WWII, when the allied powers pushed back authoritarian regimes in Europe and East Asia, the U.S. quickly turned to the rapidly spreading threat of communism. In addition to the Soviet Union, it seemed in 1947 that communism was all but certain in China; it officially took over as the dominant party in China when the Chinese Civil War ended with a Communist victory in 1949. As a result, the Cold War began between the Communist and anti-Communist factions of the world.
George F. Kennan, an American diplomat, created the policy of containment. This policy authorized and encouraged America to intervene where communism was spreading with the intention of stopping communism before it spread. This was to counter the "domino theory," the idea that countries would fall to communism like dominoes. For Kennan, only containment could stop the spread of communism. Kennan's ideas about containment helped influence the Truman Doctrine by encouraging Truman to ask for aid in Greece and Turkey in order to stop the spread of communism. This policy also helps explain our policies towards Korea in the late 1940's and 1950's. Northern Korea had adopted communism from its neighbor China, and North Korea also had support from the Soviet Union, so the United States under Truman decided to "contain" the communist threat in Korea before it spread. Unfortunately, America was only successful in maintaining democracy in South Korea, and North Korea remains communist to this day.
Back home in the United States, the threat of communist manifested itself in Congress. With Senators like Joseph McCarthy railing against communists in the U.S., the Red Scare occurred throughout the 1950's and dominated American politics and culture. Not only was McCarthy famous for his railing speeches, but he also went after people in the public eye that may or may not have had communist ties. Many Hollywood actors, directors, writers, and more were "blacklisted" for being communists and were no longer able to find work. People were accused of communism and some were even imprisoned as enemies of the state. McCarthy and his spiteful rhetoric faded, but the impact had been done. It was clear the U.S. was afraid of communism, which helps explain future actions with Cuba when the Cuban Communist Revolution was successful and communism took hold only a hundred or so miles south of Florida.