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The Arlington Line
When federal troops seized the Arlington heights, in May 1861, they immediately began the construction of Forts Runyon, Corcoran, Albany, and Scott for the protection of Washington. After the Union disaster at the First Battle of Manassas (Bull Run) in late July 1861, work was begun on Fort Ethan Allen, Fort Richardson, and a line of breastworks and lunettes called in general the Arlington Line. A similar line of redoubts and breastworks covering the valley of Four Mile Run was more properly part of the defenses of the main base of the Army of the Potomac at Alexandria. The permanent garrison of these extensive fortifications within the present bounds of Arlington County, at least 10,000 men, greatly out-numbered the resident population, then some 1,400 men, women, and children.
The Arlington Line was never attacked, not even after the federal defeat at the Second Battle of Manassas (Bull Run) — which means that it served effectively its strategic purpose.
See a list of forts and military installations in Arlington.