The system of coastal defenses built by the federal government after the War of 1812 was more than a series of forts standing guard over a watery frontier. It was an integrated and comprehensive plan of national defense developed by the U.S. Army Corps of Engineers, and it represented the nation's first peacetime defense policy. Known as the 'Third System' since it replaced two earlier attempts, it included coastal fortifications, but also denoted the values of the society that created it. The governing defense policy was one that combined permanent fortifications to defend seaports, a national militia system, and a small regular army. The Third System remained the defense paradigm in the United States from 1816 to 1861, when the onset of the Civil War changed that paradigm. In addition to providing the country with military security, the system also provided the context for the ongoing debates in Congress over national defense through the annual congressional debates on military funding. In charting the philosophy, construction, and maintenance of the forts of the Third System, a history of antebellum American national defense policy emerges, and permits an investigation of the political, economic, social, and military development of the U.S. between the War of 1812 and the Civil War.