Notes from Kloss, William, et al. Art in the White House: A Nation's Pride. Washington, D.C.: The White House Historical Association, 2008:
" . . . [T]his painting . . . almost certainly depicts an episode of October 20, 1861 [during the Civil War], that had serious consequences for the Army of the Potomac. It occurred near Leesburg, Virginia, some 40 miles upriver from Washington. Hoping to cause the Confederates to withdraw by making them think a full-scale attack was imminent, Gen. George B. McClellan made a suggestion to division commander Gen. Charles P. Stone on the Maryland side of the river: 'Perhaps a slight demonstration on your part would have the effect to move them.' That afternoon at Edwards Ferry, east of Leesburg and just below Harrison's Island, Stone had his artillery fire several shells across the river.

"The casualness in Thompson's rendering of the scene reflects the reality. That only one cannon seems to be firing indicates the desultory nature of the maneuver. . . . Members of the drum corps lounge about, and the air of nonengagement is in keeping with the softly painted autumnal centerpiece of the scene, pumpkins scattered among corn shocks. . . . All the details are subordinated to the panoramic sweep of the landscape, itself calm and peaceful despite the puff of smoke from the cannon. . . .

"Although the action depicted occurred in October 1861, the painting . . . must have been executed much later from sketches, since Thompson left America before the end of 1861."