Fort Hamilton is a United States Army installation in the southwestern corner of the New York City borough of Brooklyn, surrounded by the communities of Bay Ridge and Dyker Heights. It is one of several posts that are part of the region which is headquartered by the Military District of Washington. Its mission is to provide the New York metropolitan area with military installation support for the Army National Guard and the United States Army Reserve. The original fort was completed in 1831, with major additions made in the 1870s and 1900s. However, all defenses except about half of the original fort have been demolished or buried.