The day’s events unfolded in a way that was vastly different from what has generally been portrayed by historians over the years. The first player to take the stage was Union spy Thomas Harter.

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Realizing the importance of his knowledge of the Confederates’ plan, Harter left their camp on the morning of August 18, swam the Rapidan and found his way to Reno’s headquarters. Generals Pope, McDowell, Reno and others were present when he arrived. According to McDowell, Harter reported that “the enemy had
accumulated all his force, including several divisions just up from Richmond, behind the ridge [Clark's Mountain] immediately beyond the river and opposite our extreme left.” In addition, the spy reported that the Confederates’ artillery horses were harnessed and that the troops were ready to cross the Rapidan at Raccoon Ford to get in the rear of Pope’s army. Their movement into this position, according to McDowell, “had been completely hidden from our sight by the ridge, and even from that of our lookouts on the top of Thoroughfare Mountain, was one made in the direction which had been expected from the first, and had for its object the interposing of the whole of the enemy’s forces between our army and its re-enforcements.” That information was timely and would be of use, said McDowell, “provided the enemy gave us the night and day the start.” This was more than an understatement; Harter’s report to Pope may well have been the timeliest single product of espionage received by any Union commander during the entire war.

At sunrise on August 18, Colonel Duryee and 250 men of the 2nd Maryland Infantry reached the summit of Clark’s Mountain on their raid to disrupt the operations of the Confederate signal corpsmen, which was using the summit of the mountain as a signaling station. A short skirmish with the small Rebel force manning the post ensued, and two members of the 2nd Maryland were wounded. Two Confederates were taken prisoner. Also captured were several signal flags, code books and other papers. After securing the area, the Union detachment lingered long enough to allow a topographical engineer to make observations. The view from the mountain that morning presented an unpleasant surprise–Confederate troops so close that the raiders could hear drums in the camp below pounding out the “Long Roll.”

Observations completed, the detachment hurried back to their camp, spurred on by the knowledge that the Confederates would not be far behind. They stumbled on a shorter route via Somerville Ford that cut several miles off their march.

Recognizing the importance of the topographical engineer’s detailed observations, Duryee sent him ahead with an escort. According to Duryee, the following report was written and sent to Pope at about 7:30 a.m., after the engineer had reached Reno and before the remainder of the detachment, with prisoners in tow, had returned: “I sent, under command of Lieutenant Colonel Duryea [sic], Second Regiment Maryland Volunteers, a force of 250 men to break up the rebel signal station on Clark’s Mountain. The expedition left at 1 p.m. [a.m.] and arrived at the summit at day light next morning. A small mounted force was found there, and a slight skirmish took place in which several of the enemy were wounded and 2 captured. As soon as Colonel Duryea [sic] arrives I will send the prisoners to headquarters. They captured a signal flag and a memorandum book, from which it appears that Jackson’s Army is back of Clark’s Mountain, probably in the vicinity of Orange Court House. I send herewith the book and other papers. The cavalry has not yet returned.”

In addition to Harter, another Union spy, Richard Montgomery, had infiltrated the Confederate army. Leaving McDowell’s headquarters on the 17th, he spent the evening of the 17th and most of the 18th with the enemy force. He returned to the Union lines on the evening of the 18th. During his stay he learned that the Confederates had been reinforced and were about to make an attempt to cross the Rapidan upstream from the Union position.

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