Isoroku Yamamoto was a Marshal Admiral of the Imperial Japanese Navy (IJN) and the commander-in-chief of the Combined Fleet during World War II. Yamamoto held several important posts in the Imperial Navy, and undertook many of its changes and reorganizations, especially its development of naval aviation. He was the commander-in-chief during the early years of the Pacific War and oversaw major engagements including the attack on Pearl Harbor and the Battle of Midway. Yamamoto was killed in April 1943 after American code breakers identified his flight plans, enabling the United States Army Air Forces to shoot down his aircraft.

The Combined Fleet was the main sea-going component of the Imperial Japanese Navy. Until 1933, the Combined Fleet was not a permanent organization, but a temporary force formed for the duration of a conflict or major naval maneuvers from various units normally under separate commands in peacetime.