The Guadalcanal campaign, also known as the Battle of Guadalcanal and codenamed Operation Watchtower by American forces, was a military campaign fought between 7 August 1942 and 9 February 1943 on and around the island of Guadalcanal in the Pacific theater of World War II. It was the first major land offensive by Allied forces against the Empire of Japan.

On 7 August 1942, Allied forces, predominantly United States Marines, landed on Guadalcanal, Tulagi, and Florida in the southern Solomon Islands, with the objective of using Guadalcanal and Tulagi as bases in supporting a campaign to eventually capture or neutralize the major Japanese base at Rabaul on New Britain. The Japanese defenders, who had occupied those islands since May 1942, were outnumbered and overwhelmed by the Allies, who captured Tulagi and Florida, as well as the airfield later named Henderson Field that was under construction on Guadalcanal.

Surprised by the Allied offensive, the Japanese made several attempts between August and November to retake Henderson Field. Three major land battles, seven large naval battles (five nighttime surface actions and two carrier battles), and almost daily aerial battles culminated in the decisive Naval Battle of Guadalcanal in early November, with the defeat of the last Japanese attempt to bombard Henderson Field from the sea and to land enough troops to retake it. In December, the Japanese abandoned their efforts to retake Guadalcanal, and evacuated their remaining forces by 7 February 1943, in the face of an offensive by the U.S. Army's XIV Corps, with the Battle of Rennell Island, the last major naval engagement, serving to secure protection for the Japanese troops to evacuate safely.

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 BunaGona, 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 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 (19441945), and the Volcano and Ryukyu Islands campaign prior to the surrender of Japan in August, 1945.

The Allies, later known formally as the United Nations, were an international military coalition formed during the Second World War (1939–1945) to oppose the Axis powers, led by Nazi Germany, Imperial Japan, and Fascist Italy. Its principal members by 1941 were the United Kingdom, United States, Soviet Union, and China.

Membership in the Allies varied during the course of the war. When the conflict broke out on 1 September 1939, the Allied coalition consisted of the United Kingdom, France, and Poland, as well as their respective dependencies, such as British India. They were soon joined by the independent dominions of the British Commonwealth: Canada, Australia, New Zealand and South Africa. Consequently, the initial alliance largely resembled that of the First World War.

As Axis forces began invading northern Europe and the Balkans, the Allies added the Netherlands, Belgium, Norway, Greece, and Yugoslavia. The Soviet Union, which initially had a nonaggression pact with Germany and participated in its invasion of Poland, joined the Allies in June 1941 after Operation Barbarossa. The United States, while providing some materiel support to European Allies since September 1940, remained formally neutral until the Japanese bombing of Pearl Harbor in December 1941, after which it declared war and officially joined the Allies. China had already been at war with Japan since 1937, but formally joined the Allies in December 1941.

The Allies were led by the so-called "Big Three"—the United Kingdom, the Soviet Union, and the United States—which were the principal contributors of manpower, resources, and strategy, each playing a key role in achieving victory. A series of conferences between Allied leaders, diplomats, and military officials gradually shaped the makeup of the alliance, the direction of the war, and ultimately the postwar international order. Relations between the United Kingdom and the United States were especially close, with their bilateral Atlantic Charter forming the groundwork of the alliance.

The Allies became a formalized group upon the Declaration by United Nations on 1 January 1942, which was signed by 26 nations around the world; these ranged from small nations far removed from the war, to governments in exile from the Axis occupation. The Declaration officially recognized the Big Three and China as the "Four Powers", acknowledging their central role in prosecuting the war; they were also referred to as the "trusteeship of the powerful", and later as the "Four Policemen" of the United Nations. Many more countries joined through to the final days of the war, including colonies and former Axis nations.

After the war ended, the Allies, and the Declaration that bound them, would become the basis of the modern United Nations; one enduring legacy of the alliance is the permanent membership of the U.N. Security Council, which is made up exclusively of the principal Allied powers that won the war.