On March 30, 1942, US
Admiral Chester Nimitz was appointed Commander-in-Chief,
Pacific Ocean Areas. In the other major theater in the Pacific region, known as the
South West Pacific theater, Allied forces were commanded by US
General Douglas MacArthur. Both Nimitz and MacArthur were overseen by the US
Joint Chiefs, and indirectly by the British-American
Combined Chiefs of Staff. Most Japanese forces in the theater were part of the of the
Imperial Japanese Navy (IJN), which was responsible for all Japanese warships,
naval aircraft, and
marine infantry units. The
Rengō Kantai was led by Admiral
Isoroku Yamamoto, until he was killed in an
attack by U.S. fighter planes in April 1943. Yamamoto was succeeded by Admiral
Mineichi Koga (1943–44) and Admiral
Soemu Toyoda (1944–45). The of the
Imperial Japanese Army (IJA) was responsible for Imperial Japanese Army ground and air units in
Southeast Asia and the South Pacific. The IJN and IJA did not formally use joint/combined staff at the operational level, and their command structures/geographical areas of operations overlapped with each other and those of the Allies. In the Pacific Ocean theater, Japanese forces fought primarily against the
United States Navy, the
U.S. Army, which had 6 Corps and 21 Divisions, and the
U.S. Marine Corps, which had only 6 Divisions. The
United Kingdom (
British Pacific Fleet),
New Zealand, Australia,
Canada, and other Allied nations, also contributed forces. == Major campaigns and battles ==