Benson J. Lossing, Pictorial Field Book of the Civil War. Volume 2.

Pope saw that he had no alternative. He must fight. So he put his line in V shape early the next morning, pivoting on the Warrenton pike. Reynolds occupied the left leg, Porter, Sigel, and Reno the right, and Heintzelman was posted on the extreme right. Pope had resolved to attack Lee's left, and at the same time the latter had made disposition during the night to attack Pope's left. Lee's movements for that purpose, in which he withdrew some of his troops from ground he had occupied the previous evening, gave Pope the impression that his foe was retreating along the Warrenton pike, and he was not undeceived until ten o'clock the next day. Meanwhile he had telegraphed to Washington the joyful tidings that the Confederates were “retreating to the mountains.” under this impression he ordered McDowell to follow with three corps, Porter's in the advance, along the Warrenton pike, and attack the fugitives, and Heintzelman and Reno, supported by Ricketts' division, were directed to assail and turn the Confederate left.

the attempt to execute this movement developed a fearful state of affairs for the National Army. As Butterfield's division moved up the Hill near Groveton, the eminence near the edge of the woods suddenly and unexpectedly swarmed with the Confederates, who, instead of retreating, had been massing under cover of the forest in preparation for an offensive movement. They at once opened a fierce fire of shot, shell, and bullet on the Nationals, and at the same time clouds of dust on the left indicated that the foe, in great numbers, were making a flank movement in that direction. To meet this peril McDowell ordered Reynolds to leave Porter's left, and hasten to the assistance of Schenck and Milroy, on whom the threatened blow seemed about to fall. This exposed Porter's key-point, when Colonel G. K. Warren, without orders, moved up with his little brigade of a thousand men and took Reynolds's place. Ricketts, in the mean time, had hastened to the left, and the battle soon became very severe. Porter's corps, which had been made to recoil by the force of the First unexpected blow, was rallied, and performed special good service, especially Warren's gallant little band of volunteers, and a brigade of regulars under Colonel Buchanan. For a while victory seemed to incline to the Nationals, for Jackson's advanced line was steadily pushed back until about five o'clock in the afternoon. Then Longstreet turned the tide. He found a commanding point on Jackson's right, and with four batteries he poured a most destructive raking artillery fire upon the Nationals. Line after line was swept away, and very soon the whole left was put to flight. Jackson immediately advanced, and Longstreet moved in support by pushing his heavy columns against Pope's center. Hood, with his two brigades, charged furiously upon Ricketts and Reynolds, followed by the divisions of Evans, R. H. Anderson, and Wilcox, supported by those of Kemper and Jones, and at the same time Lee's artillery was doing fearful execution on Pope's disordered infantry. Terrible was the struggle until dark, when it ceased. The National left had been pushed