There was a strong fear of communism spreading throughout Europe because of the perceived threat it posed to capitalist societies and democratic governments. The Soviet Union's success in spreading communism in Eastern Europe, as well as the communist revolution in China, added to the fear that communism would continue to spread and threaten the Western world.
The domino theory was the belief that if one country fell to communism, it would lead to a domino effect where other countries would also fall to communism. This theory influenced U.S. foreign policy during the Cold War, as it led to the U.S. becoming more involved in foreign affairs and supporting anti-communist governments around the world.
The Marshall Plan was a U.S. aid program that provided financial assistance to European countries to rebuild after World War II. The plan was used as a containment strategy because it aimed to prevent the spread of communism by providing economic assistance to countries that were vulnerable to communist influence. The Marshall Plan helped to rebuild Europe's economy and helped to create a stronger, more stable Europe that was less susceptible to communist influence.