In 1904 he converted to Catholicism, and later went on to be the first Roman Catholic co-chairman of the
National Conference of Christians and Jews alongside Everett Clinchy and Roger Williams Strauss, serving as co-chairman 1928 to 1946 despite a 1928 papal encyclical which explicitly prohibited such ecumenical interactions. He was chairman of Columbia's History department several times. After World War I, he joined with
Peter Guilday in establishing the American Catholic Historical Association and became its first secretary. Its goal was to promote Catholic history and to integrate Catholic scholars into the wider academic world. Hayes was an internationalist, unlike Beard, and opposed
isolationism in the 1930s while denouncing totalitarianism. In his 1945 presidential address to the American Historical Association, titled "The American Frontier—Frontier of What?", he urged Americans to see their nation as the western frontier of Europe. The
Founders had maintained "lively contacts with, and solid knowledge of, the European civilization on whose boundaries they were". In the 19th century, with massive immigration from Europe, "Americans" took a different path from Europeans, becoming a nation of diverse linguistic, religious and ethnic origins, with each group desperate to be accepted. While nationalism in Europe emerged from an appreciation for the cultural or political achievements of one's compatriots, American nationalism encouraged fresh cultural and political developments. Hayes concluded that this had produced an intense, and often artificial form of nationalism, that served to "inoculate us against Europe and built up an isolationist state of mind". During
World War I, he served at the rank of captain in the United States Military Intelligence Division of the
General Staff from 1918 to 1919. Nine years later, under the direction of
General Connor, the head of the
War Department, he was asked to serve on an advisory committee of historians to organize documents pertaining to the American participation in the fighting in France. This earned him the title of major. In the 1930s, he was a member of the
Catholic Association for International Peace. He served a term as president of the
American Historical Association in 1945 and was head of the
New York State Historical Association in
Cooperstown. He was also a member of the
American Philosophical Society. Hayes was awarded the
Laetare Medal from
Notre Dame in 1946, the
Alexander Hamilton medal from
Columbia University in 1952, and the
Gibbons Medal from
The Catholic University of America in 1949. He was a guest lecturer and teacher at various academic institutions throughout his career and into his retirement and was given honorary degrees from the following institutions: •
University of Notre Dame, 1921 •
Marquette University, 1929 •
Niagara College, 1936 •
Williams College, 1939 •
Fordham University, 1946 •
University of Detroit, 1950 •
Georgetown University, 1953 •
Michigan State University, 1955 •
LeMoyne College, 1960 ==Ambassador to Spain==