Fort Strong is a former U.S. Army Coast Artillery fort that occupied the northern third of Long Island in Boston Harbor. The island had a training camp during the American Civil War, and a gun battery was built there in the 1870s. The fort was built in 1893-1906 during the Endicott period of expansion in U.S. coast defense, and was part of the Coast Defenses of Boston. Formerly, it was also known as Long Island Military Reservation. Before World War I, a large station for handling submarine mines was added to the fort's defenses. Prior to World War I the fort was probably manned by over 1,000 soldiers. During World War II, two batteries of 3-inch guns defended channel minefields, but the big guns and other 3-inch batteries were decommissioned. Declared surplus in 1947, the fort served as a Nike missile site until 1961, and was redeveloped in 2005-2009 for a children's summer camp and later a homeless shelter. However, in 2014 the island's bridge was demolished due to safety concerns, and all activity ended.

The modern era of defending American harbors with controlled mines or submarine mines began in the post-Civil War period, and was a major part of US harbor defenses from circa 1900 to 1947.