Promoted to rear admiral in 1940, Kusaka served as chief of staff of the First Air Fleet and helped to plan the attack on Pearl Harbor. He also assisted the commander of the Combined Fleet, Admiral Yamamoto Isoroku, plan the attack on Midway. He participated in this operation as chief of staff of the First Air Fleet.
Kusaka fought in the October 1942 Battle of the Santa Cruz Islands during the Guadalcanal Campaign as chief of staff of the Third Fleet. In 1944, he became chief of staff of the Combined Fleet under Admiral Toyoda Soemu, participating in the Battles of the Philippine Sea and Leyte Gulf. At the very end of the war, in August 1945, he was appointed commander of the Fifth Air Fleet. Kusaka died at Hyogo, Japan, on 23 November 1971. Kotani Ken
Evans, David C., and Mark R. Peattie. Kaigun: Strategy, Tactics and Technology in the Imperial Japanese Navy, 1889–1941. Annapolis, MD: Naval Institute Press, 1997.; Kusaka, Ryunosuke. Ichi Kaigunshikan no Hanseiki (Life of a naval officer). Tokyo: Kowado, 1973.