He served as midshipman on the cruiser Matsushima on its long-distance navigational training cruise to Honolulu, Hilo, Wellington, Brisbane, Palm Island, Queensland, Batavia, Singapore, Mako, Qingdao, Port Arthur, Dairen, Chemulpo, Chinkai, Busan and Kagoshima.

After further attendance at Japan's Naval War College, Koga held shore staff posting following his graduation and promotion to lieutenant commander in 1917.

On his promotion to captain on December 1, 1926, Koga was again posted to France, where he served as a naval attaché in Paris until November 1, 1928.

With the start of the Pacific War, Koga commanded naval operations during the Battle of Hong Kong from December 9, 1941 to the end of the month.

[2] However, the losses of Japan's land and carrier based aircraft in the Southwest Pacific eventually forced a Japanese withdrawal from the Solomons to the aero-naval base complex at Rabaul in the Bismarcks by the end of the year, and the simultaneous defeat in the Gilbert Islands of the Central Pacific in November 1943.

His Chief of staff, Vice Admiral Shigeru Fukudome, survived ditching in a second airplane off Cebu and was held captive by Filipino guerrillas, during which time Koga's battle plans found their way into Allied hands.