The Harbor Defenses of the Delaware was a United States Army Coast Artillery Corps harbor defense command. It coordinated the coast defenses of the Delaware River estuary from 1897 to 1950, beginning with the Endicott program. These included both coast artillery forts and underwater minefields. The areas protected included the cities of Philadelphia, Camden, and Wilmington along with the Chesapeake & Delaware Canal. The command originated circa 1896 as an Artillery District and became the Coast Defenses of the Delaware in 1913, with defenses initially at and near Fort Delaware on Pea Patch Island near Delaware City. In 1925 the command was renamed as a Harbor Defense Command. During World War II the defenses were relocated to Fort Miles on Cape Henlopen at the mouth of the Delaware Bay.

Fort Delaware is a former harbor defense facility, designed by chief engineer Joseph Gilbert Totten and located on Pea Patch Island in the Delaware River. During the American Civil War, the Union used Fort Delaware as a prison for Confederate prisoners of war, political prisoners, federal convicts, and privateer officers. A three-gun concrete battery of 12-inch guns, later named Battery Torbert, was designed by Maj. Charles W. Raymond and built inside the fort in the 1890s. By 1900, the fort was part of a three fort concept, the first forts of the Coast Defenses of the Delaware, working closely with Fort Mott in Pennsville, New Jersey, and Fort DuPont in Delaware City, Delaware. The fort and the island currently belong to the Delaware Department of Natural Resources and Environmental Control (DNREC) and encompass a living history museum, located in Fort Delaware State Park.