From 1916 to 1922, Koga held a number of administrative assignments ashore, including that of resident officer in France. Promoted to commander in 1922 and to captain in 1926, he was naval attaché to France between 1926 and 1928 and attended the 1927 Geneva Arms Limitation Conference. Returning to Japan in 1928, he was then secretary to the minister of the navy and a member of the Japanese delegation to the London Naval Conference of 1930. From 1930 to 1932, he commanded first the heavy cruiser Aoba and then the battleship Ise. Promoted to rear admiral in 1932, Koga next held a number of staff positions, including head of the Intelligence Division (1933) and vice chief of the Naval General Staff (1937). He was promoted to vice admiral in 1936.
Koga commanded Second Fleet from 1939 to 1941. He strongly opposed the conclusion of the Tripartite Pact with Germany and Italy in September 1940. In 1941, he assumed command of the China Area Fleet, supporting naval operations against Hong Kong at the beginning of the Pacific war. Promoted to full admiral in May 1942, Koga then commanded the Yokosuka Naval Station (1942–1943).
In May 1943, Koga succeeded Yamamoto Isoroku as commander in chief of the Combined Fleet following the latter's death in the Solomon Islands on 18 April 1943. By mid-1943, Japan had lost Guadalcanal as well as Attu and Kiska in the Aleutians to Allied counteroffensives. Koga worked to rebuild Japanese naval air strength and at the same time sought to retrieve the situation before it became irreversible by a decisive naval action employing the Combined Fleet. In October 1943, Koga ordered Vice Admiral Ozawa Jisaburo to launch the ro-go Operation to attack U.S. naval forces in the Solomons, but Ozawa suffered a major reversal in the operation, losing 120 of 170 aircraft in November.
Koga then planned the o-go Operation to smash U.S. naval forces in the Marshall Islands in February 1944. In the process of relocating his headquarters from Palau to Davao in the southern Philippines for this operation, Koga's plane was lost in a heavy storm on 31 March 1944. He was posthumously promoted to Admiral of the Fleet. Admiral Toyoda Soemu succeeded Koga as commander of the Combined Fleet. 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.; Howarth, Stephen. Morning Glory: The Story of the Imperial Japanese Navy. London: Hamish Hamilton, 1983.