Japanese control of the western Pacific area between May and August 1942. Guadalcanal
is located in the lower right center of the map.
On 7 December 1941, Japanese forces attacked the United States Pacific Fleet at Naval Station Pearl Harbor, incorporated territory of Hawaii. The attack killed almost 2,500 people and crippled much of the U.S. battleship fleet, precipitating an open and formal state of war between the two nations the next day. The initial goals of Japanese leaders were to neutralize the U.S. Navy, seize possessions rich in natural resources, and establish strategic military bases to defend Japan's empire in the Pacific Ocean and Asia. To further those goals, Japanese forces captured the Philippines, Thailand, Malaya, Singapore, Burma, the Dutch East Indies, Wake Island, Gilbert Islands, New Britain and Guam. Joining the U.S. in the war against Japan were the rest of the Allied powers, several of whom, including the United Kingdom, Australia, and the Netherlands, had also been attacked by Japan.
The Japanese made two attempts to continue their strategic initiative, and offensively extend their outer defensive perimeter in the south and central Pacific to where they could threaten Australia and Hawaii or the U.S. West Coast. Those efforts were thwarted at the naval battles of Coral Sea and Midway respectively. Coral Sea was a tactical stalemate, but a strategic Allied victory which became clear only much later. Midway was not only the Allies' first clear major victory against the Japanese, it significantly reduced the offensive capability of Japan's carrier forces, but did not change their offensive mindset for several crucial months in which they compounded mistakes by moving ahead with brash, even brazen decisions, such as the attempt to assault Port Moresby over the Kokoda trail. Up to this point, the Allies had been on the defensive in the Pacific but these strategic victories provided them an opportunity to take the initiative from Japan.
The Allies chose the Solomon Islands (a protectorate of the United Kingdom), specifically the southern Solomon Islands of Guadalcanal, Tulagi and Florida Island, as the first target, designated Task One (codenamed Pestilence), with three specific objectives. Originally, the objectives were the occupation of the Santa Cruz Islands (codenamed Huddle), Tulagi (codenamed Watchtower), and "adjacent positions". Guadalcanal (code name Cactus), which eventually became the focus of the operation, was not even mentioned in the early directive and only later took on the operation-name Watchtower.
The Imperial Japanese Navy (IJN) had occupied Tulagi in May 1942 and had constructed a seaplane base nearby. Allied concern grew when, in early July 1942, the IJN began constructing a large airfield at Lunga Point on nearby Guadalcanal—from such a base Japanese long-range bombers would threaten the sea lines of communication from the West Coast of the Americas to the populous East Coast of Australia. By August 1942, the Japanese had about 900 naval troops on Tulagi and nearby islands and 2,800 personnel (2,200 being Korean forced laborers and trustees as well as Japanese construction specialists) on Guadalcanal. These bases would protect Japan's major base at Rabaul, threaten Allied supply and communication lines and establish a staging area for a planned offensive against Fiji, New Caledonia and Samoa (Operation FS). The Japanese planned to deploy 45 fighters and 60 bombers to Guadalcanal. In the overall strategy for 1942 these aircraft could provide air cover for Japanese naval forces advancing farther into the South Pacific.
The Allied plan to invade the southern Solomons was conceived by U.S. Admiral Ernest King, Commander in Chief, United States Fleet. He proposed the offensive to deny the use of the islands by the Japanese as bases to threaten the supply routes between the United States and Australia and to use them as starting points. With U.S. President Franklin D. Roosevelt's tacit consent, King also advocated the invasion of Guadalcanal. Because the United States supported Great Britain's proposal that priority be given to defeating Germany before Japan, the Pacific theater had to compete for personnel and resources with the European theater.
An early obstacle was a desire by both the army and Roosevelt to initiate action in Europe. In addition, it was unclear who would command the campaign: Tulagi lay in the area under the command of General Douglas MacArthur, whereas the Santa Cruz Islands lay in Admiral Chester W. Nimitz's Pacific Ocean Area, which would also supply almost all offensive forces that would prepare and be supplied and covered from that area. Both problems were overcome, and the Chief of Staff of the U.S. Army, General George C. Marshall, gave the operation his full support, even if MacArthur's command could not lend support and the navy had to take full responsibility. As a result, and in order to preserve the unity of command, the boundary between MacArthur's Southwest Pacific area and Nimitz's Pacific Ocean area was shifted 60 miles (97 km) to 360 miles (580 km) to the west effective 1 August 1942.
The Chairman of the Joint Chiefs of Staff established the following objectives for 1942–1943: that Guadalcanal would be taken, in conjunction with an Allied offensive in New Guinea under Douglas MacArthur, to capture the Admiralty Islands and the Bismarck Archipelago, including the major Japanese base at Rabaul. The directive held that the eventual goal was the American reconquest of the Philippines. The U.S. Joint Chiefs of Staff created the South Pacific theater, with Vice Admiral Robert L. Ghormley taking command on 19 June 1942, to direct the offensive in the Solomons. Admiral Chester Nimitz, based at Pearl Harbor, was designated as overall Allied commander in chief for Pacific forces.
The airfield at Lunga Point
under construction by Japanese and conscripted Korean laborers in July 1942
In preparation for the offensive in the Pacific in May 1942, U.S. Marine Major General Alexander Vandegrift was ordered to move his 1st Marine Division from the United States to New Zealand. Other Allied land, naval and air force units were sent to establish or reinforce bases in Fiji, Samoa, New Hebrides and New Caledonia.
Espiritu Santo, New Hebrides, was selected as the headquarters and the main base for the offensive, codenamed Operation Watchtower, with the commencement date set for 7 August 1942. At first, the Allied offensive was planned just for Tulagi and the Santa Cruz Islands, omitting Guadalcanal. After Allied reconnaissance discovered the Japanese airfield construction efforts on Guadalcanal, its capture was added to the plan and the Santa Cruz operation was (eventually) dropped. The Japanese were aware, via signals intelligence, of the large-scale movement of Allied forces in the South Pacific area but concluded that the Allies were reinforcing Australia and perhaps Port Moresby in New Guinea.
The Watchtower force, numbering 75 warships and transports (of vessels from the U.S. and Australia), assembled near Fiji on 26 July 1942 and engaged in one rehearsal landing prior to leaving for Guadalcanal on 31 July. The commander of the Allied expeditionary force was U.S. Vice Admiral Frank Fletcher, Commander Task Force 61 (whose flag was on the aircraft carrier USS Saratoga). Commanding the amphibious forces was U.S. Rear Admiral Richmond K. Turner. Vandegrift led the 16,000 Allied (primarily U.S. Marine) infantry earmarked for the landings.
The troops sent to Guadalcanal were fresh from military training and armed with bolt-action M1903 Springfield rifles and a meager 10-day supply of ammunition. Because of the need to get them into battle quickly, the operation planners had reduced their supplies from 90 days to only 60. The men of the 1st Marine Division began referring to the coming battle as "Operation Shoestring".