The vast majority of Allied forces in the theatre were from the US Navy and US Marine Corps. However units and/or personnel from New Zealand, the United Kingdom, Australia, Canada, Mexico, Fiji and other countries also saw active service.
On 24 March 1942, the newly-formed British and US Combined Chiefs of Staff issued a directive designating the Pacific theater an area of American strategic responsibility. Six days later the US Joint Chiefs of Staff (JCS) divided the Pacific theater into three areas: the Pacific Ocean Areas (POA), the Southwest Pacific Area (SWPA), and the Southeast Pacific Area.
The JCS designated Admiral Chester W. Nimitz as Commander in Chief, Pacific Ocean Areas (CINCPOA), with operational control over all units (air, land, and sea) in that area. Mainland Asia was excluded from the POA, as were the Philippines, the Netherlands East Indies, the Territory of New Guinea (including the Bismarck Archipelago) and the western part of the Solomon Islands. US strategic bomber forces in the theatre were under the direct control of the US Joint Chiefs of Staff. All land forces in Alaska and Canada remained under the control of the US Army's Western Defense Command (see World War II: Aleutian Islands).
The Joint Chiefs further divided the Pacific Ocean Areas into the North, Central, and South Pacific Areas. Nimitz designated subordinate commanders for the North and South Pacific Areas (NORPAC and SOPAC) but retained the Central Pacific Area (CENPAC), including the Hawaiian Department, under his direct command.
In the separate South West Pacific Area, General Douglas MacArthur assumed command. The effective result of this organizational scheme was the creation of two separate commands in the Pacific: POA and SWPA, each reporting separately to the Joint Chiefs, each competing for scarce resources in an economy-of-force theater, and each headed by a commander in chief (CINC) from a different service. In particular, the division of the Solomons caused problems, since the battles of the Solomon Islands campaign in 1942–1943 ranged over the whole region, with the main Japanese bases in SWPA and the main Allied bases in SOPAC.