The Dutch Harbor Naval Operating Base and Fort Mears were the two military installations built next to each other in Dutch Harbor, on Amaknak Island of the Aleutian Islands in Alaska, by the United States in response to the growing war threat with Imperial Japan during World War II. In 1938, the Navy Board recommended the construction which began in July 1940. The first United States Army troops arrived in June 1941 and an air base constructed by the United States Navy was finished in September 1941. At the time of the surprise attack on Pearl Harbor, these were the only military installations in the Aleutian Islands.