Fort Corcoran was a wood-and-earthwork fortification constructed by the Union Army in northern Virginia as part of the defenses of Washington, D.C. during the American Civil War.

[1] Fort Corcoran was home to the Union Army Balloon Corps and the headquarters of the defenses of Washington south of the Potomac River, and served throughout the war before being dismantled in 1866.

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.

Engineer officers under the command of then-Colonel John G. Barnard accompanied the army and began building fortifications and entrenchments along the banks of the Potomac River in order to defend the bridges that crossed it.

The 69th New York, part of the famed Irish Brigade, went about its work with high spirits, exemplified by the actions of Father Thomas Mooney, the 69th's chaplain.

By the time Barnard was beginning to focus his efforts on tying Corcoran and Runyon into an entire belt of fortifications, his engineers were drawn off by the approach of the Confederate Army and the incipient First Battle of Bull Run.

[12] Following the Union defeat at Bull Run, panicked efforts were made to strengthen the forts built by Barnard in order to defend Washington from what was perceived as an imminent Confederate attack.

On July 23, President Abraham Lincoln and Secretary of State William Seward visited Fort Corcoran in an effort to revive morale after the defeat at Bull Run.

[21] Throughout the remainder of the war, Fort Corcoran remained an important communications center for the entire Arlington Line system of defenses, eventually connecting to over 30 miles of telegraph wire at the time of Robert E. Lee's surrender.

[30] He was only at Fort Corcoran for a short time before he and the rest of his command were withdrawn from the defenses of Washington in order to counter Lee's advance into Maryland.

[32] Replacing Porter was Brigadier General Gustavus Adolphus DeRussy, whose division was spread across multiple forts in the Arlington Line defenses.

As did Porter before him, DeRussy made his headquarters at Fort Corcoran and directed the defense of the Arlington Line until after the final surrender of Confederate forces.

From September 1861 through August 1863, Fort Corcoran served as the official headquarters for the Union Army Balloon Corps, a civilian organization commanded by Thaddeus S. C. Lowe, who had been appointed to the position by President Lincoln.

Over the next few days, he repeatedly ascended in the balloon to make maps of the surrounding area and gather intelligence about Confederate troops advancing on Washington.

Beginning August 30, 1861, Lowe's new balloon made daily ascents to monitor the movements of Confederate troops that had moved north following the Battle of Bull Run.

[38] By September 11, 1861, General F.J. Porter, in command of the forces in the area of Fort Corcoran was impressed enough by Lowe's daily reports to write, "You are of value now.

[40] The agreed-upon signaling system was simple, but effective: "If we fire to the right of Falls Church, let a white flag be raised in the balloon; if to the left, let it be lowered; if over, let it be shown stationary; if under, let it be waved occasionally.

[44] Men at the fort served in regimental bands or played baseball in their spare time between periods of infantry and gunnery drill.

[citation needed] Much of the work at the fort dealt with maintenance and renovation of the original structure, which had been hurriedly thrown up in the wake of the Union advance into Arlington after the secession of Virginia.

An 1864 report by Brigadier General Richard Delafield, chief of the U.S. Army Corps of Engineers lists an extensive series of improvements made to the fort over the previous year.

On March 7, 1866, Edward N. Lucas, a local resident, complained that "he was assaulted and badly beaten by colored soldiers from Fort Corcoran at the Aqueduct Bridge yesterday.

[47][48][49] After it became apparent that the forces under the command of Confederate general Jubal Early were moving further north, the additional troops at Fort Corcoran marched northward to counter them.

[51] After the surrender of General Robert E. Lee's Army of Northern Virginia on April 9, 1865, the primary reason for manned defenses protecting Washington ceased to exist.