General McDowell's army was moved up to and concentrated around the ridge on which Centerville is situated during the 18th and 19th, with intent to advance and attack the enemy, posted along Bull Run and between that stream and Manassas junction, on Saturday, the 20th. But delay was encountered in the reception of adequate subsistence, which did not arrive till Friday night. On Saturday three days' rations were distributed and issued, and every preparation was made for moving punctually at two o'clock next morning. Meantime General P. T. Beauregard, maintaining an absolute quiet and inoffensiveness on his front, and fully informed by spies and others of every movement between him and Washington, had hastily gathered from every side all the available forces of the Confederacy, including fifteen thousand, or nearly the full strength, of General Joseph E. Johnston's Army of the Shenandoah, and had decided to assume the offensive and attack our forces before General Robert Patterson could come up to join them. Had the advance been made on Saturday, as originally intended, it would have encountered but two-thirds of the force it combated; had it been delayed a few hours longer, the Federal troops would have stood on the defensive, with the immense advantages of knowing the ground and of choosing the positions whereon to fight. Such are the casualties and fatalities of war.
Bull Run is a decent mill-stream, fordable in summer at intervals of half a mile to a mile. Its immediate valley is generally narrow and wooded, enclosed by bluffs, neither high nor very steep, but affording good positions for planting batteries to command the roads on the opposite side, so screened by woods and brush as to be neither seen nor suspected until the advancing or attacking party is close upon them. This fact explains and justifies General McDowell's (or Scott's) order of battle. This was, briefly: to menace the enemy's right by the advance of our First division on the direct road from Centerville to Manassas Junction, while making a more serious demonstration on the road running due west from Centerville to Groveton and Warrenton, and crossing Bull Run by the Stone Bridge; while the real or main attack was to be made by a column fifteen thousand strong, composed of the Second (David Hunter's) and Third (Samuel P. Heintzelman's) divisions, which, starting from their camps a mile or two east and southeast of Centerville, were to make a considerable detour to the right, crossing Cub Run, and then Bull Run, at a ford known as Sudley Spring, three miles above the Stone Bridge, thus turning the enemy's left, and rolling it up on the centre, where it was to be taken in flank by our First division (Tyler's) crossing the Stone Bridge at the right moment, and completing the rout of the enemy. The Fifth division (D. J. Miles's) was held in reserve at Centerville, not only to support the attacking columns, but to guard against the obvious peril of a formidable Confederate advance across Blackburn's Ford to Centerville, flanking our flank movement, capturing munitions and supplies, and cutting off the line of retreat. The Fourth division (Runyon's) guarded communications with Alexandria and Arlington; its foremost regiment being about seven miles from Centerville.
The movement of the Federal army was appointed for 2.30 A.M., and the battle should have been opened at 6 A.M.; but the raw troops never had been brigaded before this advance, and most of their officers were without experience; so that there was a delay of two or three hours in the flanking divisions reaching the point at which the battle was to begin. General Tyler, in front of Stone Bridge, opened with his artillery at 6.30 A.M., eliciting no reply; and it was three hours later when Hunter's advance, under Colonel Ambrose E. Burnside, crossed at Sudley Spring; his men, thirsty with their early march that hot July morning, stopping as they crossed to drink and to fill their canteens.