Stonewall Jackson, having seized the attention of the Union Army of Virginia commander, US General John Pope, now needed to pick a fight on favorable ground, and prevent the Union army from escaping to the safety of the defense of Washington, DC. At the same time, he needed to position himself so as to allow the other half of the Confederate Army of Northern Virginia, with Lee and Longstreet to link up with him.
It is for that reason that Jackon, after seizing the railroad Junction at Manassas, marched his Corps north during the night of 27-28 August to the Unfinished Railroad grade along Stony Ridge -- next to the battlefield of First Manassas. This was good ground, and was a relatively short march from Thoroughfare Gap, through which Lee and Longstreet would have to lead their Corps to reach Jackson (they had been a day behind Jackson in departing, but progressed at a slower rate).
Union General John Pope, however, saw a great opportunity to crush this isolated part of the Confederate army, and sought an chance to "bag" the famed Stonewall Jackson. He gave orders for his widely scattered Corps to join at the high ground of Centreville, and from there deal with Jackson. Pope didn't actually know precisely where Stonewall Jackson had gone, but heard reports of his men headed for Centreville.
It was in this situation that the III Corps, US Army of Virginia, of US General Irvin McDowell found itself playing a crucial role on that 28 August 1862. His III Corps would find itself in two key places that day -- next to the essential ground of Thoroughfare Gap, and on the line of march past Jackson's Corps to Centreville. These would be the two fights of this day.
McDowell, sensing the strategic importance of the gap in the Bull Run Mountains known as Thoroughfare Gap, ordered one of his divisions, that of US General James Rickett's, to keep an eye on it. Another division, that of US General Rufus King, he ordered down the Warrenton Turnpike towards Centreville. Both divisions would see action!
Sign marking the center of the Battle of Thoroughfare Gap. You can read the full text and find the location here: http://www.hmdb.org/marker.asp?marker=606
At Thoroughfare Gap, McDowell's concerns were validated when, that afternoon, CS General James Longstreet's Corps of Robert E. Lee's Army of Northern Virginia arrived to join Stonewall Jackson -- and they had to pass through the Gap. Rickett's Union Division, of only about 5,000 men, tried to block over 20,000 Confederates. The bottleneck of the Gap, however, did allow this small Union force to delay the Southern advance, but it could do no more than delay. Eventually Confederate forces gained the heights and passed through a gap to the north, Hopewell Gap, and Ricketts was forced to fall back. US General Pope seemed little concerned with this development, however, and unfortunately for Ricketts, his division never received help.
You can read the NPS account of the fight at the Battle of Thoroughfare Gap here: http://www.cr.nps.gov/hps/abpp/battles/va025.htm
The sign marking the opening of the fight at Brawner Farm. This link provides the full text and location, which is at the Manassas National Battlefield Park: http://www.hmdb.org/marker.asp?marker=45952
Meanwhile, that same afternoon and evening, King's Division of the Union III Corps, on the march to Centreville, were set upon by Stonewall Jackson's forces near Groveton, Virginia in the immediate vicinity of the Brawner Farm. Essentially, Stonewall Jackson, trying to prevent the Union army from joining at Centreville, a rather invulnerable position, picked a fight with Union General Pope by revealing his location near Groveton and picking a fight with King's Division.
Map of the fighting at Brawner Farm, between the Divisions of CS Gen. Ewell and US Gen. King. [Map by Hal Jespersen, www.cwmaps.com]
The fighting at Brawner Farm, sometimes considered its own battle and a prelude to Second Manassas, and sometimes the first day of Second Manassas, was brutal, intense, and only came to a close with darkness that evening of 28 August 1862. It was on this field that the Confederate Stonewall Brigade fought to a draw with the Union Iron Bridge. The two sides pounded each other into the night at less than 100 yards distance. CS General Richard Ewell would lose a leg in the action at Brawner Farm. That afternoon and evening felled 1,150 Union troops and 1,250 Confederates, killed or wounded.
You might note the website of the action at Brawner Farm, that of the Manassas National Battlefield Park: http://www.nps.gov/mana/index.htm
The old Brawner farm house has been restored, and there is a splendid display on the Second Battle of Manassas -- including a fibre-optic battle map -- located in the house. It is a must-visit if you go to Manassas Battlefield!