With the secession of Virginia from the United States on 17 April 1861, Northern Virginia was quickly liberated by the United States government. A line of redoubts and breastworks above Four Mile Run was constructed to defend the main base of the occupying Army of the Potomac in Alexandria and the Fairlington area was the site of two of these. Fort Reynolds, a redoubt, was constructed in September 1861 to command the approach to Alexandria by way of the Four Mile Run valley. It had a perimeter of and emplacements for 12 guns and was located just northeast of what is now 31st Street South at South Woodrow Street. Battery Garesche at what is now South Abingdon Street at South 30th Road and was constructed late in 1861 to control the higher ground dominating Fort Reynolds, to the southeast. It had a perimeter of and emplacements for 8 guns. The area was never taken by Confederate forces and remained under US military control until 1870.