The Guadalcanal campaign, also known as the Battle of Guadalcanal and codenamed Operation Watchtower by the United States, was an Allied offensive against forces of the Empire of Japan in the Solomon Islands during the Pacific Theater of World War II. It was fought between 7 August 1942 and 9 February 1943, and involved major land and naval battles on and surrounding the island of Guadalcanal. It was the first major Allied land offensive against Japan during the war.

In summer 1942, the Allies decided to mount major offensives in New Guinea and the Solomon Islands with the objectives of defending sea lines to Australia and eventually attacking the major Japanese base at Rabaul on New Britain. The Guadalcanal operation was under the command of Robert L. Ghormley, reporting to Chester W. Nimitz, while the Japanese defense consisted of the Combined Fleet under Isoroku Yamamoto and the Seventeenth Army under Harukishi Hyakutake.

On 7 August 1942, Allied forces, predominantly U.S. Marines, landed on Guadalcanal, Tulagi, and Florida Island in the southern Solomon Islands. The Japanese defenders, who had occupied the islands since May 1942, offered little initial resistance, but the capture of Guadalcanal soon turned into a lengthy campaign as both sides added reinforcements. The Allies captured and completed Henderson Field on Guadalcanal and established a defense perimeter. The Japanese made several attempts to retake the airfield, including in mid-September and in late October. The campaign also involved major naval battles, including the Battles of Savo Island, the Eastern Solomons, Cape Esperance, and the Santa Cruz Islands, culminating in a decisive Allied victory at the Naval Battle of Guadalcanal in mid-November. Further engagements took place at the Battle of Tassafaronga and Battle of Rennell Island. In December, the Japanese decided to abandon Guadalcanal to focus on the defense of the other Solomon Islands, and evacuated their last forces by 9 February 1943.

The campaign followed the successful Allied defensive actions at the Battle of the Coral Sea and the Battle of Midway in May and June 1942. Along with the battles at Milne Bay and Buna–Gona on New Guinea, the Guadalcanal campaign marked the Allies' transition from defensive operations to offensive ones, and effectively allowed them to seize the strategic initiative in the Pacific theater from the Japanese. The campaign was followed by other major Allied offensives in the Pacific, most notably: the Solomon Islands campaign, New Guinea campaign, the Gilbert and Marshall Islands campaign, the Mariana and Palau Islands campaign, the Philippines campaign of 1944 to 1945, and the Volcano and Ryukyu Islands campaign prior to the surrender of Japan in August 1945.

On 7 December 1941, Japanese forces attacked the United States Pacific Fleet at Pearl Harbor, Hawaii. The attack killed almost 2,500 and crippled much of the U.S. battleship fleet, precipitating formal declarations of war between the two nations the next day. The initial goals of the Japanese leadership were to neutralize the U.S. Navy, seize territories rich in natural resources, and establish strategic military bases with which to defend Japan's empire in the Pacific Ocean and Asia. Initially, Japanese forces captured the Philippines, Thailand, Malaya, Singapore, Burma, the Dutch East Indies, Wake Island, Gilbert Islands, New Britain and Guam. The U.S. was joined in the war against Japan by several of the Allied powers, including the British Empire and the Dutch government-in-exile, both of which had also been attacked by Japan.

The Japanese made two attempts to continue their offensive and extend their outer defensive perimeter in the south and central Pacific to a point at which they could threaten Australia, Hawaii, and the U.S. west coast. The first offensive was thwarted in the naval Battle of the Coral Sea, which was a tactical stalemate but a strategic Allied victory in that it checked Japan's expansion across the Pacific for the first time: it was the Allies' first major victory against the Japanese and significantly reduced the offensive capability of Japan's carrier forces.

However, the battle did not temper Japan's audacious offensive military posture for several crucial months, with Japanese forces attempting a failed attack on Port Moresby over the Kokoda track. The second major Japanese offensive was stopped at the Battle of Midway. Both sides suffered significant losses in carrier aircraft and aircrew during these engagements. Crucially, while the Americans were able to reconstitute their naval air strength in relatively short order, Japanese limitations in industrial and pilot training capacity meant combat losses rapidly outpaced replacement men and matériel. These strategic victories allowed the Allies to transition to a more offensive stance in the Pacific theater, and attempt to seize the strategic initiative from Japan.

The Allies chose the Solomon Islands (a protectorate of the United Kingdom), specifically the southern islands of Guadalcanal, Tulagi and Florida Island, as their first target, designated Task One (codename Pestilence), with the initial objectives of occupying the Santa Cruz Islands (codename Huddle), Tulagi (codename Watchtower), and "adjacent positions". Guadalcanal (codename 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. Tulagi, although small, had a large natural harbor that was ideal for a float-plane base; Florida Island also had to be taken, as it dominated Tulagi. Guadalcanal, much larger than the other two islands and located to the south across the soon-to-be-named Ironbottom Sound, was added when it was discovered the Japanese were constructing an airbase there.