For 20 of his 24 years in the Senate, Green served on the Foreign Relations Committee, beginning in 1938 and interrupted from 1947 to 1949. An early and steadfast internationalist committed to the United Nations, he stoutly sustained President Truman's Cold War initiatives, including the Truman Doctrine, the Marshall Plan, the North Atlantic Treaty Organization, and intervention in Korea. At the 1952 meeting of the UN General Assembly, to which Truman appointed him as a delegate, Green expressed his faith in the world organization as the "last great hope of mankind." He stood with the minority of 31 senators who by one vote prevented the two-thirds majority necessary to pass an amendment initiated by Senator John W. Bricker to limit the president's powers in foreign policy.