The army of the Shenandoah, when it joined Beauregard, was composed of the First brigade, four Virginia infantry regiments and Pendleton's Virginia battery, under Col. T. J. Jackson; Second brigade, three Georgia regiments, two Kentucky battalions and Alburtis' Virginia battery; Third brigade, one Alabama, two Mississippi and one Tennessee regiment, and Imboden's Virginia battery, under Brig.-Gen. B. E. Bee; Fourth brigade, one Tennessee and two Virginia regiments, a Maryland infantry battalion, and Grove's Virginia battery, under Col. A. Elzey; and one Virginia regiment of infantry and one of cavalry, not brigaded. The army of the Potomac, it was estimated, had 9,713 men of all arms engaged; the army of the Shenandoah had a total of 8,340 of all arms for duty. The combined army was estimated to contain some 30,000 men of all arms; but only about 18,000 of these were actually engaged in the battle.
When Beauregard took command at Manassas, Johnston's "army of the Shenandoah," in the lower Shenandoah valley, was, in a sense, Beauregard's left, although not under his command, as Johnston ranked him. On the right, at Aquia creek, on the Potomac, holding the terminus of the Richmond, Fredericksburg & Potomac railroad, was a Confederate force of some 2,500 men, under Brig.-Gen. T. H. Holmes. Beauregard had a small advanced outpost, under Colonel Hunton, at Leesburg, watching the fords of the upper Potomac east of the Blue ridge; another at Fairfax, in direct observation of the Federal army at Washington, with detachments on the line of the railway toward Alexandria, and to the south of that road, guarding the approaches to his right from Alexandria. The principal advantage of his chosen line of defense was that it was an interior one.
From information that he deemed authentic, Beauregard concluded that he was confronted by an army of 50,000 men, fully equipped and ready for offensive operations, under the direction, as general-in-chief, of Lieut.-Gen. Winfield Scott, then considered the most able, as he was the most distinguished military officer on the American continent, and under the immediate command of Brig.-Gen. Irvin McDowell, one of the most esteemed of the active officers of the Federal army. To oppose these
