American Cold War policy during this period was guided by the principle of containment. According to this principle, which was first proposed by George Kennan and officially articulated as the Truman Doctrine, the United States should make every attempt to check the expansionism of communism, which was conceived as a malignant, growing force. The American decision to support anti-communist fighters in Turkey and Greece, as well as the enormous recovery program for western Europe known as the Marshall Plan, were examples of non-military containment. Other examples include the Berlin Airlift to undermine the Soviet blockade of that city, and the Americans' refusal to vacate West Berlin in general. US intervention under the auspices of the United Nations on the Korean Peninsula, a decision that led to the Korean War, was an example of military containment.