Just before World War II, the U.S. War Department began upgrading the harbor defenses protecting Delaware Bay by building new fortifications at Cape Henlopen that became known as Fort Miles. The fort expanded as members of the 261st and 21st Coast Artillery Units constructed new buildings. After the war, the U.S. Army used the post for research and development, and the cantonment area became a recreational post. In 1964, the Army gave the first parcel of Fort Miles to the state of Delaware, creating Cape Henlopen State Park, and it continued turning over land until the last parcel came in 1996. At its height, the fort housed 2,300 soldiers, guns that could fire more than 25 miles out to sea, and the second largest underwater minefield along the east coast of the United States.