The collection contains approximately 145 letters to Armour, somewhat affectionate and personal in nature, from Presidents Hoover, Roosevelt, Truman, Eisenhower and Nixon.Other notable correspondents include John Foster Dulles, Dean Acheson, Henry Stimson, George Kennan and other State Department figures, various U.S. Senators, and J. Edgar Hoover, as well as heads of state and officials from foreign posts such as Haiti, Canada and Argentina.Many of the letters express appreciation for individual speeches or Armour's work in general, approval of a promotion or reassignment, or regret for his retirement.Thus, the correspondence documents periods of transition in the diplomat's career, and illustrates the respect and friendship he inspired among officials in both his assigned and home countries.The collection also contains approximately 55 carbons and hand-written drafts of letters from Armour to the aforementioned people and others.

In his letters, speeches and official reports, Armour often refers to his experiences in revolutionary Russia, which helped shape his more conservative and considered manner of diplomacy.In 1919, while stationed at the American Embassy in Petrograd, he wrote to Robert McElroy: "Bolshevism, with its appeal to all that is basest, and a programme which holds out as bait to ignorant workingmen the immediate satisfaction of all their wishes and desires, is...capable of wrecking every country, as it has already wrecked Russia....I believe it has in it the germs capable of destroying civilization itself." President Nixon later referred to the prediction in a letter to Armour: "You proved, unfortunately, to be an extremely accurate prophet at a time when very few in this country recognized the dangers ahead."

Long letters between Armour and some of his more unusual acquaintances, such as writers James Thurber, Rudyard Kipling and W. Somerset Maugham, reveal Armour's more jocular side as well as his own story-telling abilities.Letters to his father describe his new surroundings in Paris and the mundane details of making travel arrangements and renovating the flat ("We are meeting the electricians, painters, plumbers, etc. at the apartment on that day and they will then 'take possession' for another five or six weeks....").