Question: why did the usa develop a policy of containment after 1945

title: Containment
text: Truman , under the guidance of Acheson , followed up his speech with a series of measures to contain Soviet influence in Europe , including the Marshall Plan , or European Recovery Program , and NATO , a 1949 military alliance between the U.S. and Western European nations . Because containment required detailed information about Communist moves , the government relied increasingly on the Central Intelligence Agency ( CIA ) . Established by the National Security Act of 1947 , the CIA conducted espionage in foreign lands , some of it visible , more of it secret . Truman approved a classified statement of containment policy called NSC 20 / 4 in November 1948 , the first comprehensive statement of security policy ever created by the United States . The Soviet Union 's first nuclear test in 1949 prompted the National Security Council to formulate a revised security doctrine . Completed in April 1950 , it became known as NSC 68 . It concluded that a massive military buildup was necessary to deal with the Soviet threat . According to the report , drafted by Paul Nitze and others :