The Aleutian Islands campaign (アリューシャン方面の戦い) was a military campaign conducted by the United States, Canada, and Japan in the Aleutian Islands, part of the Territory of Alaska, in the American Theater and the Pacific Theater of World War II beginning with the invasion by Japanese forces on 3 June 1942. In the only two invasions of the United States during the war of a U.S. incorporated territory, a small Japanese force occupied the islands of Attu and Kiska, where 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. Successful Japanese invasions of other U.S. territories, which were unincorporated territories, in the western Pacific shortly after the Attack on Pearl Harbor included Wake Island, Guam, and the Philippines. The islands' strategic value was their ability to control Pacific transportation routes as US General Billy Mitchell stated 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 Japan proper via the Kuril Islands. Similarly, the US feared that the islands could be used as bases from which to carry out a full-scale aerial attack on West Coast cities such as Anchorage, Seattle, San Francisco, or Los Angeles.
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an invasion force landed on Kiska in the wake of a sustained three-week barrage, only to discover that the Japanese had withdrawn from the island on 29 July.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.