This has been touched on in previous posts but I wanted to flesh out a whole post on it because I think it provides really important context.
The actions of the 11th New York were one part of a much larger campaign to seize portions of Virginia across the Potomac River from Washington DC.
On a map, what we're looking at is this area:
The citizens of Virginia ratified secession on May 23, 1861. That night, Union soldiers began crossing into Virginia. Winslow Homer sketched the scene as Union soldiers crossed the Long Bridge. It appeared in the June 8, 1861 edition of Harper's Weekly:
Caption: "The advance guard of the grand army of the United States crossing the Long Bridge over the Potomac, at 2 a.m. on May 24, 1861."
Also seized on May 24, 1861 was Arlington House.
The scene was described in the New York Herald on May 25 under the headline,
"THE INSURRECTION. ADVANCE OF THE FEDERAL TROOPS INTO VIRGINIA, WASHINGTON, May 24, 1861". I bolded the references to Ellsworth's men.
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.
About ten o'clock last night four companies of picked men moved over the Long Bridge, as an advance guard. They were sent to reconnoitre, and if assailed were ordered to signal, when they would have been reinforced by a corps of regular infantry and a battery.
At twelve o'clock Colonel Ellsworth's regiment of Zouaves embarked in steamers from the Navy Yard for Alexandria, and must have reached there about one o'clock this morning. They landed under the cover of the Pawnee's guns. An attack would have been signalized. No attack was made.
At twelve o'clock the infantry regiment, artillery and cavalry corps began to muster and assume marching order. As fast as the several regiments were ready they proceeded to the Long Bridge, those in Washington being directed to take that route.
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. They took possession of the heights in that direction.
The imposing scene was at the Long Bridge, where the main body of the troops crossed. 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.
The Twelfth (New York) was the first on the ground. The army crossed the bridge in the following order:
— Twelfth regiment, New York.
— Twenty-fifth regiment, New York.
— First regiment, Michigan.
— First, Second, Third and Fourth, New Jersey, in the order named.
— Two regular cavalry corps, of eighty men each, and Sherman's two batteries.
— Next and last came the New York Seventh, the liveliest party, and with more men than any other regiment. They seemed delighted at the idea that they were to have a show at something that looked like service before returning home.
— Following them was a long train of wagons filled with wheelbarrows, shovels, &c.
Altogether there were at least thirteen thousand troops in the advancing army. This includes the Zouaves who went by steamer, the forces that moved from Georgetown, as well as the main body that proceeded over the Long Bridge.
General Mansfield commanded the movement of the troops until the last corps left the district. The first regiment of the main body that crossed the Long Bridge started at twenty minutes past two, and the last corps left the district at about a quarter to four o'clock.
At four o'clock Major General Sandford and staff left Willard's, and proceeded to Virginia to take command of the advancing forces. He informed me that he should establish his headquarters on Arlington Heights, and should take possession of the Arlington mansion.
Two thousand troops, the New York Zouaves and New York Twelfth, are to occupy Alexandria; the remainder the heights by regiments from the chain bridge to Alexandria.
General Mansfield took the greatest care to instruct the troops just before entering the bridge to take the route step-that is, to avoid marching together, as the solid step together might injure the bridge. . . .
The sun of the 24th of May has risen and exposed to our gratifying gaze the Stars and Stripes floating over Alexandria, where the secession flag has been haunting the sight for weeks past. Truly the past has been a great night's work for the Union. Secession is suddenly doomed, and nothing but an ignominious doom awaits the leading traitors in this great wrong against popular government and free institutions.
Ground was broken on May 24 for Forts Runyon and Corcoran in Virginia, the former guarding the Long Bridge and the latter the Aqueduct. The 7th New York State Militia worked on Fort Runyon while the soon-to-be-famous 69th New York built Fort Corcoran.
Major John G. Barnard, heading up the engineers, explained:
As tetes-de-pont to the Long Bridge and Aqueduct, Forts Runyon and Corcoran, the sites of which had been previously reconnoitered under my directions, were commenced at daylight on the morning, of the 24th. The same day a reconnaissance was made in the vicinity of Alexandria by Captain Wright, Engineers (now, Major-General U. S. Volunteers), and Fort Ellsworth, to secure our possession of that city, was commenced.
The only incident of violence on May 24 was the death of Ellsworth.
Source for Image:
https://www.civilwarmonitor.com/photo-essays/winslow-homers-civil-war
Source for NY Herald Report and Barnard's Report: nps.gov/parkhistory/online_books/civilwar/hrs1-4.htm