Aleutian Islands campaign in the context of "Japanese expansion (1941–1942)"

The Pacific War, sometimes called the Asia–Pacific War or the Pacific Theater, was the theater of World War II fought between the Japan and the Allies in East and Southeast Asia, the Pacific and Indian Oceans, and Oceania. It was geographically the largest theater of the war, including the Pacific Ocean theater, the South West Pacific theater, the Second Sino-Japanese War, and the brief Soviet–Japanese War, and included some of the largest naval battles in history. War between Japan and the Republic of China had begun in 1937, with hostilities dating back to Japan's invasion of Manchuria in 1931, but the Pacific War is more widely accepted to have begun in 1941, when the United States and United Kingdom were brought into the war, after being attacked by Japan.

Japan invaded French Indochina in 1940, and extended its control over the entire territory in July 1941. On 7–8 December 1941, Japan attacked the American naval base at Pearl Harbor in Hawaii; the U.S.-held Philippines, Guam, and Wake Island; and the British colonies of Malaya, Singapore, and Hong Kong, followed by a Japanese declaration of war. The Japanese achieved great success over the next 6 months, allying with Thailand and capturing the listed territories (except for Hawaii) in addition to Borneo, New Britain, the Dutch East Indies, Burma, the Solomon and Gilbert Islands, and parts of New Guinea. In May 1942, Japanese and Allied aircraft carriers fought at the Battle of Coral Sea, resulting in the retreat of a Japanese invasion force headed for Port Moresby. In June, Japan invaded the Aleutian Islands, and in the central Pacific was defeated at the Battle of Midway, considered a key turning point in the war. After this point, the Japanese experienced great difficulty replacing their losses in ships and aircraft as the U.S. produced ever increasing numbers of both.

The Battle of Midway was a major naval battle in the Pacific Theater of World War II that took place on 4–7 June 1942, six months after Japan's attack on Pearl Harbor and one month after the Battle of the Coral Sea. The Japanese Combined Fleet under the command of Isoroku Yamamoto suffered a decisive defeat by the U.S. Pacific Fleet near Midway Atoll, about 1,300 mi (1,100 nmi; 2,100 km) northwest of Oahu. Yamamoto had intended to capture Midway and lure out and destroy the U.S. Pacific Fleet, especially the aircraft carriers which had escaped damage at Pearl Harbor.

Before the battle, Japan desired to extend its Pacific defense perimeter, especially after the Doolittle air raid of Tokyo in April 1942, and to clear the seas for attacks on Midway, Fiji, Samoa, and Hawaii. A related Japanese attack on the Aleutian Islands began one day earlier, on 3 June. The Japanese strike force at Midway, known as the Kidō Butai, was commanded by Chuichi Nagumo. Yamamoto's plan for the operation, which depended on precise timing and coordination, was undermined by its wide dispersal of forces, which left the rest of the fleet unable to support the Kidō Butai effectively.