American Military History, Volume 1 by Richard W. Stewart

In the early summer of 1861 the partly trained ninety-day militia, the almost untrained volunteers, and one newly organized battalion of regulars-a total force of 50,000 Federals commanded by Brig. Gen. Irvin McDowell-defended the nation's capital. Thirty miles to the southwest, covering the rail and road hub at Manassas, Virginia, General Beauregard posted 20,000 Confederates, to be joined by 2,000 more within a few days. To the left, on their defensive line along the Potomac, the Confederates stationed another 11,000 men under Brig. Gen. Joseph E. Johnston in the Shenandoah Valley town of Winchester. Opposing Johnston around Martinsburg, with the mission of keeping the Confederates in place, was Maj. Gen. Robert Patterson with 18,000 Federals. On the extreme right of the Confederate northern Virginia defense line was Col. John B. Magruder's force, which had recently repulsed Maj. Gen. Benjamin F. Butler's Union troops at Big Bethel, Virginia, on 10 June and forced them back into their sanctuary at Fort Monroe.

Big Bethel, the first large-scale engagement of the Civil War, demonstrated that neither opponent was as yet well trained. The Confederates had started preparations earlier to protect northern Virginia and therefore might have had a slight edge on their opponents. General McDowell, only recently a major of regulars, had less than three months to weld his three types of units (militia, volunteer, and regular) into a single fighting force. He attempted to do too much himself, and there were few competent staff officers in the vicinity to help him. McDowell's largest tactical unit was a regiment until just before he marched out of Alexandria. Two to four brigades, plus a battery of regular artillery-the best arm against raw infantry-formed a division. In all, thirteen brigades were organized into five divisions. McDowell parceled out his forty-nine guns among his brigade commanders, who in turn attached them to their regiments. His total force for the advance was 35,732 men, but of these one division of 5,752 men dropped off to guard roads to the rear.