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 march, which began at 10 p.m. on the night of the 23rd, was described in colorful terms by the New York Herald two days later: There can be no more complaints of inactivity of the government.

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.

[7] 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.

There was nothing to prevent the enemy shelling the city from heights within easy range, which could be occupied by a hostile column almost without resistance.

I looked up at Fort Lyon, which at that moment went up with a tremendous shock...It...looked...like the pictures of Vesuvus [sic] during an eruption... Everything flew up from the center and seemed to stand still for a moment...then...pieces of steel, stones, and dirt, came rattling, and thundering down...[19]In the wake of the explosion, Fort Lyon, and Alexandria were visited by many military and civilian dignitaries, including Secretary of War Edwin Stanton and President Abraham Lincoln.

[22] There is also a historical marker at the south end of Mount Eagle Park describing the earthen works erected by the Union Army.