Fearing the threat from Imperial Japan during World War II, the then neutral United States started fortifying Dutch Harbor in 1940, resulting in the construction of the Dutch Harbor Naval Operating Base and Fort Mears that was finished entirely by September 1941 three months before the surprise attack on Pearl Harbor that caused the U.S. to enter the war. On June 3, 1942, the town was attacked by Japanese air forces in the Battle of Dutch Harbor, part of the Aleutian Islands Campaign. It was the first city in the continental United States to be attacked by enemy aircraft during World War II. After the attack and the Japanese occupations of Kiska and Attu, almost all of the native residents of Amaknak Island were evacuated. Many were held, under poor conditions, in camps in Southeast Alaska for the duration of the war; a substantial number of the internees died during the imprisonment.