On June 17, 1861, Union Army Maj. Gen. Irvin McDowell sent the 1st Ohio Volunteer Infantry Regiment (90–day) under the overall command of Schenck and the immediate command of Col. Alexander McDowell McCook to expand the Union position in Fairfax County, Virginia. Schenck took six companies over the Alexandria, Loudon and Hampshire Railroad line, dropping off detachments to guard railroad bridges between Alexandria, Virginia and Vienna, Virginia. As the train approached Vienna, about north of Fairfax Court House and from Alexandria, 271 officers and men remained with the train. On the same day, Confederate States Army Col. Maxcy Gregg took the 6–month 1st South Carolina Infantry Regiment, about 575 men, two companies of cavalrymen (about 140 men) and a company of artillery with two artillery pieces (35 men), about 750 men in total, on a scouting mission from Fairfax Court House toward the Potomac River. On their return trip, at about 6:00 p.m., the Confederates heard the train whistle in the distance. Gregg moved his artillery pieces to a curve in the railroad line near Vienna and placed his men around the guns. Seeing this disposition, an elderly local Union sympathizer ran down the tracks to warn the approaching train of the hidden Confederate force. The Union officers mostly ignored his warning and the train continued down the track. In the only response to the warning, an officer was placed on the forward car as a lookout.