Fortifications

Fort Runyon was a timber and earthwork fort constructed by the Union Army following the occupation of northern Virginia in the American Civil War in order to defend the southern approaches to the Long Bridge as part of the defenses of Washington, D.C. during that war. The Columbia Turnpike and Alexandria and Loudon Railroad ran through the pentagonal structure, which controlled access to Washington via the Long Bridge. With a perimeter of almost 1,500 yards (1,400 m), and due to its unusual shape it was approximately the same size, shape, and in almost the same location as the Pentagon, built 80 years later.

Runyon was built immediately after the entry of Union forces into Virginia on May 24, 1861, on the land of James Roach, a Washington building contractor.[3] Fort Runyon was the largest fort in the ring of defenses that protected Washington during the Civil War and was named after Brigadier General Theodore Runyon, commander of the Fourth Division of the Army of Northeastern Virginia during the First Battle of Bull Run. Union soldiers garrisoned the fort until its dismantling following the end of the Civil War in 1865. Today, no trace of the fort remains on the site, which is now exit 9 on Interstate 395, though a historical marker has been constructed by the Arlington Historical Society.  ( Source - Wikipedia )