Shortly after the Union Army's rout at the First Battle of Bull Run (Manassas) in late July 1861, the Army constructed in August 1861 a lunette (Fort Ramsay) on the future grounds of Fort Myer. One of the first fortifications built on the Arlington Line, the lunette was located at and near the present post's Forest Circle. Later renamed to Fort Cass, the lunette had a perimeter of and emplacements for 12 guns.