Like Roosevelt, Stimson found public service more satisfying than a career in law and soon became active in New York Republican politics. Appointed secretary of war in 1911, he followed in Root's footsteps in attempting to modernize the U.S. Army, improving troop training and the efficiency of the General Staff, although congressional opposition blocked his contemplated consolidation and rationalization of army posts around the country.
When World War I began in Europe in 1914, the staunchly interventionist and pro-Allied Stimson campaigned ardently for preparedness, massive increases in American military budgets in anticipation of war with Germany, and universal military training. After American intervention, he volunteered and served in France as a lieutenant colonel of artillery. Returning from the war, he was convinced that the United States must assume a far greater international role.
Appointed by President Herbert Hoover as secretary of state in 1929, Stimson protested firmly against Japan's 1931 establishment of the puppet state of Manzhuguo, instituting the policy of American nonrecognition of its government. In the later 1930s, he was among the strongest advocates of firm American opposition to fascist states' demands. When World War II began in Europe in 1939, Stimson, a firm believer in an Anglo-American alliance, outspokenly demanded massive American assistance to the Allies.
Although or perhaps because he was a prominent Republican, in summer 1940 Democratic President Franklin D. Roosevelt made him secretary of war, a position Stimson held until the war ended. He attracted an able group of younger lawyers and businessmen such a Robert A. Lovett, Robert P. Patterson, and John J. McCloy who not only oversaw the massive recruitment and industrial mobilization programs that the war effort demanded but also accepted and wished to carry forward the forceful internationalist tradition that their revered chief embodied.
In the spring of 1945, Stimson was the first official to inform President Harry S. Truman that his country and Britain had developed an atomic weapon. Stimson approved its use against Japan but was largely responsible for the July 1945 Potsdam Declaration, whereby the Allies first invited Japan to surrender or face attack by unspecified but highly destructive new weapons. He later published an article justifying his own and other American officials' decision to use atomic weapons against Japan on the grounds that ultimately this saved more lives than it cost. He also initially suggested that in order to disarm Soviet suspicions, the Allies should share the secrets of nuclear power with the Soviet Union, plans that ultimately proved fruitless.
After retiring in 1945, Stimson endorsed a greatly enhanced American international role, publicly supporting the Marshall Plan and the creation of the North Atlantic Treaty Organization (NATO). He published an influential volume of memoirs, setting forth his views on his country's international position. Stimson died on 20 October 1950 at Huntington, New York.
Isaacson, Walter, and Evan Thomas. The Wise Men: Six Friends and the World They Made; Acheson, Bohlen, Harriman, Kennan, Lovett, McCloy. New York: Simon and Schuster, 1986.; Schmitz, David F. Henry L. Stimson: The First Wise Man. Wilmington, DE: Scholarly Resources, 2000.; Stimson, Henry L., and McGeorge Bundy. On Active Service in Peace and War. New York: Harper, 1948.