The Marshall House in Alexandria
… and the reason this building is important is because it’s where the first Union officer was killed during the Civil War. The day after Virginia’s secession from the Union was ratified, a Confederate flag flew above the Marshall House, visible from the White House. President Lincoln was none too pleased, and his friend and army colonel, Elmer Ellsworth, offered to retrieve it.
Ellsworth was successful in traveling across the Potomac to Alexandria, uncontested, going up to the building’s roof to cut the flag down. As he came down the stairs, he was confronted by a shotgun-wielding James Jackson, who blasted Ellsworth in the chest. Elmer died instantly and was the first Union officer to be killed in the Civil War.
And here is the back of the card.