Before the outbreak of the Civil War, Alexandria County, the closest part of Virginia to Washington, D.C., was a predominantly rural area.

Originally part of the District of Columbia, the land now comprising the county was retroceded to Virginia in a July 9, 1846 act of Congress that took effect in 1847.

The U.S. Army responded by creating the Department of Washington, which united all Union troops in the District of Columbia and Maryland under one command.

The forward march movement into Virginia, indicated in my despatches last night, took place at the precise time this morning that I named, but in much more imposing and powerful numbers.

The troops quartered at Georgetown, the Sixty-ninth, Fifth, Eighth and Twenty-eighth New York regiments, proceeded across what is known as the chain bridge, above the mouth of the Potomac Aqueduct, under the command of General McDowell.

Eight thousand infantry, two regular cavalry companies and two sections of Sherman's artillery battalion, consisting of two batteries, were in line this side of the Long Bridge at two o'clock.

Throughout the remainder of the war, Alexandria would lean strongly towards the Confederate government, necessitating continued occupation by a Union garrison.

In the days that followed the Union defeat at Bull Run, panicked efforts were made to defend Washington from what was perceived as an imminent Confederate attack.

On July 26, 1861, five days after the battle, Maj. Gen. George B. McClellan was named commander of the military district of Washington and the subsequently renamed Army of the Potomac.

[16][17] Gen. John Newton, who was in charge of the forts south of Four Mile Run, supervised the construction and managed the flow of men and material.

[18] Liberated slaves, also known as "contrabands", helped build the defenses to protect Washington from invasion by Confederate forces during the Civil War.