The idea of a fort on the Penobscot long predated the U.S. government's 1844 land purchase for Fort Knox. In 1821, the Board of Engineers, established by the Secretary of War to plan the defense of the United States, identified a fort on the Penobscot as necessary though a low priority, and four years later it named the Narrows as the most effective site. This proposed fort was part of a broader coastal defense program that included more than 40 forts on the Atlantic, Gulf, and Pacific coasts. Defined in 1821 and backed by President James Monroe, the plan aimed to protect important harbors from enemy attack, deny enemies strong positions, shield cities, prevent river blockades, and safeguard coastal and interior navigation. Proposed to Congress at an estimated cost of nearly $18 million, the system was especially intended to prevent invasions like England's during the War of 1812. Using the latest engineering and armament technology, this network of forts was named the Third System by General Joseph Totten, chief of engineers and head of the Board of Engineers. Over more than 40 years, plans for the Third System included as many as 115 works, but only 42 were built. Maine had six Third System forts, and among them Fort Knox, with its main building and outlying batteries, was both the earliest and the largest in area.