The correspondence is arranged alphabetically by correspondent and chronologically within each file. It consists primarily of material relating to Hamilton Fish Armstrong's work as editor of Foreign Affairs and is extremely comprehensive. The nature of the correspondence varies widely, and includes editorial correspondence, letters debating current political issues, business letters relating to the Century Club and Armstrong's other personal interests, and personal correspondence. There is also some correspondence relating to several of Armstrong's books. As editor of Foreign Affairs, Armstrong was responsible for recruiting prominent political figures to write articles for the publication; this frequently involved domestic and foreign travel. The correspondence is comprised of 66 boxes of material.

The most notable portion of Armstrong's correspondence is with Archibald Cary Coolidge, the first editor of Foreign Affairs, 1922-1928. Coolidge had taken the position as editor on the understanding that he could remain at Harvard while Armstrong served as Managing Editor, running the New York office and taking care of the daily routine. While the correspondence generally remained on a professional level, the two were good friends and corresponded daily for more than five years. The papers include the originals of both sides of the Armstrong-Coolidge exchange as well as other Coolidge correspondence with important Council figures. There is also a small section of general correspondence belonging to Coolidge that does not relate to the Council. How these Coolidge materials came into Armstrong's hands is uncertain.

Armstrong originally learned of the position at Foreign Affairs in the early 1920's from Edwin F. Gay, who had recently become editor of The Evening Post. Gay served on a Council on Foreign Relations committee that sought to establish a journal that would address foreign policy matters. Much of Armstrong's early correspondence with Gay focuses on the creation and organization of Foreign Affairs, while the later correspondence deals primarily with maintaining and improving the financial success of the journal.

There is a significant amount of correspondence with Foreign Affairs staff members, especially Assistant Editor Byron Dexter and Managing Editor Philip W. Quigg. Both kept Armstrong up to date on Foreign Affairs matters, such as upcoming issues, possible articles, and finances when he was abroad. Essential to the routine correspondence was Armstrong's long-time secretary Mary H. Stevens, who kept things running at the office when Armstrong was away or out of the country.

One of the privileges of Armstrong's position at Foreign Affairs was the opportunity to meet influential leaders such as Dwight Eisenhower, Adolf Hitler, John F. Kennedy and Adlai E. Stevenson. An idea of the range of people with whom Armstrong corresponded on a regular basis include Dean Acheson, Frank Altschul, Newton D. Baker, Hanson Baldwin, Edvard Benes, Isaiah Berlin, General Tasker H. Bliss, Chester Bowles, Karl Brandt, Isaiah Bowman, McGeorge Bundy, William P. Bundy, Cass Canfield, Vladimir Dedijer (Yugoslav dissident), Allen W. Dulles, John Foster Dulles, Anthony Eden, Herbert Feis, Konstantin Fotitch, Felix Frankfurter, John Gunther, Bruce C. Hopper, Colonel Edward M. House, Joachim Joester, George F. Kennan, Henry Kissinger, Wolf Ladijinsky, William L. Langer, Russell C. Leffingwell, Walter Lippmann, John J. McCloy, Archibald MacLeish, Walter H. Mallory, Thomas Mann, George S. Messersmith, Francis P. Miller, J. Pierrepont Moffat, Philip E. Mosely, Gamal Abdel Nasser, Peter II (King of Yugoslavia), James B. Reston, Arthur M. Schlesinger, Jr., Bernadotte E. Schmitt, Charles Seymour, Carlo Sforza, Vincent Sheean, Harold E. Stassen, Henry L. Stimson, Dorothy Thompson, Josip Broz Tito, Jacob Viner, and Wendell L. Willkie. Please consult the container listing for a full enumeration of correspondents.

Armstrong's most prolific correspondences, however, were with public servants, academics, and journalists, discussing issues of the day, Foreign Affairs articles, and occasionally personal matters. His detailed correspondence with Madame Mabel S. Grouitch is among the more substantial correspondences of the series. Grouitch, who is best known for her work with the Serbian Aid Fund, the American Home for Yugoslav Children, and the American Yugoslav Society (files in the Organizations and Committees series), met Armstrong in 1912 after she presented a speech at Princeton University. The two became friends and corresponded regularly for over fifty years. During that time, Armstrong wrote many articles and letters in support of Grouitch's activities in Yugoslavia. Conversely, Grouitch encouraged Armstrong's endeavors with Foreign Affairs and elected him to the Executive Committee of the Serbian Aid Fund.

Each correspondent's principal interests are reflected in his/her correspondence with Armstrong. In fact, Armstrong had many of them write articles in these fields for Foreign Affairs. For example, Jay Allen discussed the Spanish Refugees who were flooding into France in many of his letters. Allen and Armstrong suggested and edited articles for each other on this topic. This exchange of ideas also worked on a personal level. For instance, Felix Frankfurter and Armstrong shared thoughts about Council dinners, Foreign Affairs articles, government policies, and international events.

Editorial correspondence was of two types–within the framework of a larger correspondence or related to a single article with no further development. Most of Armstrong's correspondence with heads of state falls under the latter category. Occasionally detailed letters regarding the editing of an article were exchanged, and these have been retained. There are very few manuscripts in the collection, and the ones that are tend to be in foreign languages. Armstrong apparently discarded the manuscript articles once they were published.

Box 5 and boxes 9-11 of the series contain correspondence related to Armstrong's articles and books. Included are notes, sources, letters with publishers, reviews, promotional materials, selected entries from Armstrong's journal, acknowledgements and contracts.

This series also contains a limited quantity of personal correspondence between members of the Armstrong family, but it is fragmentary at best, and mostly relates to Armstrong's earlier years. Armstrong pulled together most of this family correspondence while writing Those Days. Especially notable is the lack of correspondence or materials relating to Armstrong's later personal life.

Almost all social correspondence has been discarded, unless it was included in an otherwise important letter. Many telegrams were discarded because their content was either trivial or repeated elsewhere. Most published matter was also excluded; it consisted primarily of clippings from the New York Times. When notable published items were found, they were given citations on their accompanying correspondence.