Promoted to captain in 1918, Nagano commanded a cruiser in 1919 and 1920. He was the naval attaché to the United States from 1920 to 1923 and visited the United States again in 1927 and in 1933. Promoted to rear admiral in 1928, Nagano commanded the Naval Academy from 1928 to 1930. He headed the Japanese delegation at the London Naval Conference of 1935–1936, where he proposed equality of total naval tonnage with the British and U.S. navies, a proposal the western powers rejected.
In 1936, Nagano became minister of the navy in the Hirota Koki cabinet. In 1937, he was appointed commander in chief of the Combined Fleet. From April 1941 to February 1944, Nagano served as chief of the Naval General Staff, in which position he tended to follow recommendations presented by his subordinates. Nagano believed that war with the United States was inevitable and he thus had little interest in U.S.-Japanese negotiations to avoid it. At the same time, although Nagano believed the Japanese navy could not defeat the U.S. Navy in war, he did not oppose the plans of Admiral Yamamoto Isoroku, commander of the Combined Fleet, to attack Pearl Harbor.
In April 1943, Nagano was promoted to admiral of the fleet. After the Japanese surrender, he was arrested and jailed in Sugamo Prison in Tokyo. He was charged as a war criminal before the International Military Tribunal for the Far East. He died on 5 January 1947 before the end of the trial. Kotani Ken
Evans, David C., and Mark R. Peattie. Kaigun: Strategy, Tactics and Technology in the Imperial Japanese Navy, 1936–1941. Annapolis, MD: Naval Institute Press, 1997.; Marder, Arthur J. Old Friends, New Enemies. Oxford, UK: Clarendon Press, 1981.; Prange, Gordon W., with Donald Goldstein and Katherine V. Dillon. At Dawn We Slept: The Untold Story of Pearl Harbor. New York: McGraw-Hill, 1981.