William Swinton, Campaigns of the Army of the Potomac

The execution of this plan was set on foot three hours after midnight of the 20th, when the troops, breaking camp at Centreville, launched on their novel adventure, and, in a dewy moonlight night, took up the march destined to bring them into presence of the enemy. The divisions had been ordered to march at half-past 2 A. M., with the view of getting on the ground early in the morning of the 21st. Tyler's division had the advance on the main road from Centreville; and, as the two divisions under Hunter and Heintzelman, to which was intrusted the turning movement, had to follow on this road up to the point where they were to diverge to the right, it was especially urgent that no obstruction should bar their march. Nevertheless, there was delay in getting Tyler's division out of camp and on to the road, and delay in its advance, which, of course, retarded the turning column. Then the road over which Hunter and Heintzelman had to pass was found to be longer than was expected; so that, instead of getting into position by six in the morning, it was, as will subsequently appear, nine before this column debouched on the thither side of Bull Run, at Sudley's Spring. Tyler, meanwhile, had pushed on, and, by six, drew up his division in front of Stone Bridge, where he opened an artillery fire on the enemy on the opposite side of Bull Run.

While the columns of McDowell were thus under way, events of equal moment were passing within the Confederate camp. General Johnston in person had joined Beauregard during the night of the 20th (his troops, however, not having yet arrived), and, being the ranking officer, he assumed command of all the Confederate forces. Nevertheless, as Beauregard knew his ground, the plans he had formed were adopted, and Johnston directed their execution under him. This plan contemplated an offensive movement before