The Civil War Defenses of Washington were a group of Union Army fortifications that protected the federal capital city, Washington, D.C., from invasion by the Confederate States Army during the American Civil War (see Washington, D.C., in the American Civil War). The sites of some of these fortifications are within a collection of National Park Service (NPS) properties that the National Register of Historic Places identifies as the Fort Circle. The sites of other such fortifications in the area have become parts of state, county or city parks or are located on privately owned properties. A trail connecting the sites is part of the Potomac Heritage Trail.