By the summer of 1861, the Union recognized Darnestown as an ideal location for a major division headquarters because it stood at the intersection of roads leading to the Chesapeake and Ohio Canal and to Washington, D. C., and lay not far from the fords of the Potomac River. President Lincoln and his generals feared that if Confederate troops crossed the Potomac River into Montgomery County, they might invade the nation's capital from the north. During the Civil War, the rural valley of Darnestown and its citizens saw both Union and Confederate movements. Federal forces established a main line of defense and headquarters there for three brigades of General McClellan's Army of the Potomac under Major General Nathaniel P. Banks, and the encampment lasted from August to December 1861. The arrival of an entire Union division tested Darnestown's patience, loyalties, and resources as 18,000 Union troops, doubling Montgomery County's population, camped on farmland around Darnestown at Seneca Creek and in smaller encampments from Harpers Ferry to Muddy Branch. Banks' Union regiments at Darnestown in 1861 included the 9th New York State Militia, also known as the 83rd New York Infantry, the 19th New York Infantry, the 28th New York Infantry, the 46th Pennsylvania Infantry, Collis' Zouaves, also known as the 114th Pennsylvania Volunteers, the 5th Connecticut Infantry, the 1st Maryland Infantry, the 16th Indiana Infantry, the 27th Indiana Infantry, the 3rd Wisconsin Infantry, the 2nd Massachusetts Infantry, the 13th Massachusetts Infantry, and Company A, Rhode Island Light Battery.