After the Confederate Army routed the Union Army at the First Battle of Bull Run (Manassas) in late July 1861, the Union Army began construction on a line of breastworks and lunettes to the west of the earlier fortifications. These and larger fortifications later constructed nearby became known as the Arlington Line. They included a lunette (Fort Cass) and Fort Whipple, which became parts of Fort Myer, later to be renamed as Joint Base Myer–Henderson Hall.