In the late 1930s, Knox firmly believed that the United States could not remain aloof from the increasingly critical situation in Europe. Consequently, though he was unsympathetic toward President Franklin D. Roosevelt's domestic policies, he strongly endorsed the president's interventionist and pro-Allied international outlook. In 1940, Roosevelt persuaded Knox, together with former Republican Secretary of State Henry L. Stimson, to join his cabinet, with Knox serving as secretary of the navy—a step designed to win support from nonisolationist Republicans. Knox quickly recruited as assistants various able young businessmen and lawyers, such as James V. Forrestal (whom he appointed undersecretary), Ferdinand Eberstadt, and Adlai E. Stevenson. Knox utilized their industrial and organizational skills to implement expeditiously a massive naval expansion, as the U.S. Navy prepared for war in both the Atlantic and the Pacific. He also introduced modern business and management methods to the Navy Department's administration.
Knox helped to devise and lobby for the Destroyers-for-Bases deal of 1940, whereby Britain acquired American warships in exchange for leases to Caribbean naval bases, and the 1941 Lend-Lease program to aid the Allies. Like other Roosevelt administration officials, he did not predict the surprise Japanese attack on Pearl Harbor of 7 December 1941, and he rather ascribed responsibility for this U.S. defeat almost solely to the unpreparedness of the base's naval and military commanders.
Knox, who had previously found Roosevelt's pro-Allied policies insufficiently bold, welcomed American intervention in World War II. During its course, he traveled extensively to the various theaters of war. As a former newspaperman, he strongly emphasized the importance of good public relations, holding frequent press conferences. He stalwartly supported the abortive National Service Act of 1944, which would have imposed the obligation of national service on all Americans, military and civilian alike. He died suddenly of heart failure in Washington, D.C., on 28 April 1944.
Albion, Robert. Makers of Naval Policy, 1798–1947. Annapolis, MD: Naval Institute Press, 1980.; Furer, Julius Augustus. Administration of the Navy Department in World War II. Washington, DC: U.S. Government Printing Office, 1959.; Lobdell, George H., Jr. "A Biography of Frank Knox." Ph.D. diss., University of Illinois, 1954.; Lobdell, George H., Jr. "Frank Knox, 11 July 1940–28 April 1944." In Paolo E. Coletta, ed., American Secretaries of the Navy, vol. 2, 677–727. Annapolis, MD: Naval Institute Press, 1982.