Consists of both personal and public papers of Armstrong (Princeton Class of 1916), including correspondence, notebooks, memoranda, writings, memorabilia, photographs, and clippings. The correspondence series is a major resource for the shaping of 20th-century American foreign policy. It documents the history of the Council, the expanding role of FOREIGN AFFAIRS magazine, the interactions of Armstrong and Archibald Cary Coolidge in shaping the journal, and Armstrong's extended discussions with public servants, academics, and journalists regarding leading issues between 1920 and 1972. Correspondents include Dean Acheson, Jay Allen, Frank Altschul, Newton D. Baker, Hanson Weightman Baldwin, Sir Isaiah Berlin, Edvard Benes, Tasker H. Bliss, Chester Bowles, Isaiah Bowman, Karl Brandt, McGeorge Bundy, William P. Bundy, Cass Canfield, Archibald Cary Coolidge, Vladimir Dedijer, Byron Dexter, Allen and John Foster Dulles, Anthony Eden, Herbert Feis, Konstantin Fotitch, Felix Frankfurter, Mabel S. Grouitch, John Gunther, Bruce C. Hopper, Edward Madell House, Joachim Joesten, George F. Kennan, Henry Kissinger, Wolf Ladejinsky, William L. Lander, R. C. Leffingwell, Walter Lippman, Archibald MacLeish, Walter Hampton Mallory, Thomas Mann, John Jay McCloy, George S. Messersmith, Francis Pickens Miller, Jay Pierrepont Moffat, Philip E. Moseley, Gamal Abdel Nasser, Petar II Karadordevic, Philip W. Quigg, James Reston, Gaetano Salvemini, Arthur M. Schlesinger, Jr., Bernadotte E. Schmitt, Charles Seymour, Carlo Sforza, Vincent Sheean, Edward Stassen, Mary H. Stevens, Henry L. Stimson, Dorothy Thompson, Josip Broz Tito, Jacob Viner, and Wendell L. Willkie.

Other series document Armstrong's principal interests: philanthropic work for Yugoslavia; the War and Peace Studies of the Council on Foreign Relations during World War II; the President's Advisory Committee on Political Refugees; the State Department's Advisory Committee on Postwar Foreign Policies; and the United Nations Conference on International Organization (UNCIO) in San Francisco, where Armstrong served as special adviser to Secretary of State Stettinius. Armstrong's journals in the Notebooks and Memoranda and in the UNCIO series provide detailed observations on politics, world events, meetings, and interviews. In addition, there are a large number of photographs of significant public figures, Council events, the American Home for Jugoslav Children, and the UNCIO.