He was born in 1893 in the New York brownstone house near Washington Square where he lived all his adult life, a member of Edith Wharton's settled, circumscribed world of ordered privilege whose affluent, well-travelled, and sophisticated men and women traced their lineage back to the Founding Fathers and their principles to the American Revolution. His father was an artist who served as Consul General to Italy, and Armstrong was brought up in a milieu which took for granted the fact that there existed a world outside the United States. He died in 1973, as the United States finally withdrew from the Vietnam War, a conflict which deeply distressed him and shattered the foreign policy elite and its controlling consensus, whose creation had been a major part of his life's work. In an obituary notice Arthur M. Schlesinger, Jr., described him as “a New York gentleman of a vanishing school,” who “treated every one, old or young, famous or unknown, with the same generous courtesy and concern.”