American historian, educator, diplomat, devout Catholic and academic. '

He was a leading and pioneering specialist on the study of nationalism. He was elected as president of the American Historical Association over the opposition of liberals and the more explicit Anti-Catholic bias that defined the academic community of his era.

He served as United States Ambassador to Spain in World War II. Although he came under attack from the CIO, communists and other forces on the left that rejected any dealings with Francoist Spain, Hayes succeeded in his mission to keep Spain neutral during the war.