Appointed assistant secretary of state by Roosevelt in 1933, Welles implemented the administration's Good Neighbor policy of greater equality with Latin American nations. In 1937, Roosevelt promoted him to undersecretary, annoying Secretary of State Cordell Hull, who resented Welles's close relationship with the president. When World War II began in Europe, Welles won Latin American nations' acquiescence in the declaration of a neutral zone around the entire continent.
In February and March 1940, Roosevelt dispatched Welles on a well-publicized peace mission to Rome, Berlin, Paris, and London, the major European belligerent capitals, purportedly to explore the possibility of negotiating peace before Germany launched its anticipated spring offensive and, if this proved impossible, to detach Italian dictator Benito Mussolini from German leader Adolf Hitler. Another objective of this unsuccessful venture was almost certainly to demonstrate to American voters that the president had made every possible effort to bring about peace.
Welles attended Roosevelt's August 1941 meeting with Winston L. S. Churchill off the Canadian coast, where he helped to draft the Atlantic Charter. In January 1942, shortly after the Axis powers declared war on the United States, he met with all Latin American foreign ministers at Rio de Janeiro, winning their consent to a proclamation recommending a break with Germany, Italy, and Japan. For the sake of maintaining unanimity, Welles accepted this measure rather than the outright declaration of a diplomatic break preferred by Hull.
From 1939 onward, Welles was heavily involved in postwar planning undertaken jointly by the State Department and the Council on Foreign Relations. After the United States entered the war, he began an extensive speaking campaign to win popular support for the creation of a postwar international organization, which would eventually become the United Nations. In 1943, Hull, prompted by his antagonism toward Welles, used reports of his subordinate's involvement in a homosexual incident as a reason to insist on his resignation. Welles retired that September. He then wrote and spoke extensively on foreign affairs, publishing four books. He died in Bernardsville, New Jersey, on 24 September 1961.
Gellman, Irwin F. Secret Affairs: Franklin Roosevelt, Cordell Hull, and Sumner Welles. Baltimore, MD: Johns Hopkins University Press, 1995.; Graff, Frank Warren. Strategy of Involvement: A Diplomatic Biography of Sumner Welles. New York: Garland Publishing, 1988.; Welles, Benjamin. Sumner Welles: FDR's Global Strategist. New York: St. Martin's Press, 1997.; Welles, Sumner. Seven Decisions That Shaped History. New York: Harper and Brothers, 1950.