The Aleutian Islands campaign 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 starting on June 3, 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 control of the Aleutians would prevent a possible US attack across the Northern Pacific. Similarly, the US feared that the islands would be used as bases from which to carry out a full-scale aerial attack on US West Coast cities like Anchorage, Seattle, San Francisco, or Los Angeles.

A battle to reclaim Attu was launched on May 11, 1943 and completed after a final Japanese banzai charge on May 29. On August 15, 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an invasion force landed on Kiska in the wake of a sustained three-week barrage, only to discover that the Japanese had withdrawn from the island on July 29.

The campaign is known as the "Forgotten Battle" because it has been overshadowed by other events in the war.Military historians often 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 US 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.