The Aleutian Islands campaign (Japanese: アリューシャン方面の戦い, romanized: Aryūshan hōmen no tatakai) was a military campaign fought between 3 June 1942 and 15 August 1943 on and around the Aleutian Islands in the American Theater of World War II during the Pacific War. It was the only military campaign of World War II fought on North American soil. At the time of World War II, Alaska was a territory of the United States.

The islands' strategic value was their ability to control Pacific transportation routes. U.S. General Billy Mitchell told to the U.S. Congress in 1935, "I believe that in the future, whoever holds Alaska will hold the world. I think it is the most important strategic place in the world." The Japanese reasoned that their control of the Aleutians would prevent a possible joining of forces by the Americans and the Soviets and future attack on the Japan mainland via the Kuril Islands. Similarly, the U.S. feared that the islands could be used as bases from which to launch air raids on West Coast cities such as Anchorage, Seattle, San Francisco, or Los Angeles.

Following two aircraft carrier-based attacks on the American naval base at Dutch Harbor, the Imperial Japanese Navy occupied the islands of Attu and Kiska in June 1942. The remoteness of the islands and the challenges of weather and terrain delayed a larger American-Canadian force sent to eject them for nearly a year. A battle to reclaim Attu was launched on 11 May 1943 and completed after a final Japanese banzai charge on 29 May. On 15 August 1943 an invasion force landed on Kiska in the wake of a sustained three-week barrage, only to discover that the Japanese had withdrawn from the island on 29 July. The campaign is known as the "Forgotten Battle" because it has been overshadowed by other events in the war.

Many military historians believe that the Japanese invasion of the Aleutians was a diversionary or feint attack during the Battle of Midway that was meant to draw out the U.S. Pacific Fleet from Midway Atoll, as it was launched simultaneously under the same commander, Isoroku Yamamoto. Some historians have argued against that interpretation and believe that the Japanese invaded the Aleutians to protect their northern flank and did not intend it as a diversion.

Before the Empire of Japan entered World War II, the Imperial Japanese Navy had gathered extensive information about the Aleutians but had no up-to-date information regarding military developments on the islands. Admiral Isoroku Yamamoto provided the Japanese Northern Area Fleet, commanded by Vice Admiral Boshirō Hosogaya, with a force of two non-fleet aircraft carriers, five cruisers, twelve destroyers, six submarines, and four troop transports, along with supporting auxiliary ships. With that force, Hosogaya was to launch an air attack against Dutch Harbor then follow with an amphibious attack upon Adak Island, 480 miles (770 km) to the west. Hosogaya was instructed to destroy whatever American forces and facilities were found on Adak, but the Japanese did not know the island was undefended. Hosogaya's troops were to return to their ships and become a reserve for two additional landings: the first on Kiska, 240 miles (390 km) west of Adak, the other on the Aleutians' westernmost island, Attu, 180 miles (290 km) west from Kiska.

Because the Office of Naval Intelligence had broken the Japanese naval codes, Admiral Chester Nimitz learned by May 1942 of Yamamoto's plans, including the Aleutian invasion, the strength of both Yamamoto's and Hosogaya's fleets, and Hosogaya's plan to attack the Aleutians on 1 June or shortly thereafter.

As of 1 June, the U.S. military strength in Alaska stood at 45,000 men, with about 13,000 at Cold Bay (Fort Randall) on the tip of the Alaska Peninsula and at two Aleutian bases: Dutch Harbor on Unalaska Island, 200 miles (320 km) west of Cold Bay, and the recently built Fort Glenn Army Air Base on the island of Umnak 70 miles (110 km) west of Dutch Harbor. Army strength, less air force personnel, at those three bases totaled no more than 2,300, composed mainly of infantry, field and anti-aircraft artillery troops, and a large construction engineer contingent, which was used in the construction of bases. The Army Air Force's Eleventh Air Force consisted of 10 B-17 Flying Fortress heavy bombers and 34 B-18 Bolo medium bombers at Elmendorf Airfield, and 95 P-40 Warhawk fighters divided between Fort Randall and Fort Glenn. The forward headquarters was set up at Fort Geely, while the rear units were stationed at Fort Richardson. The naval commander was Rear Admiral Robert A. Theobald, commanding Task Force 8 afloat, who as Commander North Pacific Force (ComNorPac) reported to Nimitz in Hawaii. Task Force 8 consisted of five cruisers, thirteen destroyers, three tankers, six submarines, as well as naval aviation elements of Fleet Air Wing Four.

When the first signs of a possible Japanese attack on the Aleutians were known, the Eleventh Air Force was ordered to send out reconnaissance aircraft to locate the Japanese fleet reported heading toward Dutch Harbor and attack it with bombers, concentrating on sinking Hosogaya's two aircraft carriers. Once the enemy planes were removed, Naval Task Force 8 would engage the enemy fleet and destroy it. On the afternoon of 2 June, a naval patrol plane spotted the approaching Japanese fleet, reporting its location as 800 nautical miles (1,500 km; 920 mi) southwest of Dutch Harbor. Eleventh Air Force was placed on full alert. Shortly thereafter bad weather set in, and no further sightings of the fleet were made that day.