Following the Union Army's defeat at the Second Battle of Bull Run (Manassas) in August 1862, the Army constructed Fort Whipple on the grounds of the former Arlington estate during the spring of 1863. The fort was located a short distance southeast of Fort Cass. The Army named the fort after Brevet Major General Amiel Weeks Whipple, who died in May 1863 of wounds received during the Battle of Chancellorsville. The fort was considered to be one of the strongest fortifications erected for the defense of Washington during the Civil War. It had a perimeter of 658 yards and places for 43 guns.