During the Battle of First Bull Run, the principal adversaries
were the Union Army of Northeastern Virginia and two Confederate commands, the Army of the Potomac and Army of
the Shenandoah. In July 1861 the Union army was organized into five infantry divisions of three to five brigades each.
Each brigade contained three to five infantry regiments. An artillery battery was generally assigned to each brigade. The
total number of Union troops present at the Battle of First Bull Run was about 35,000, although only about 18,000 were actually
engaged.
The Confederate Army of the Potomac was organized into
six infantry brigades, with each brigade containing three to six infantry regiments. Artillery batteries were assigned to
various infantry brigades. The total number of troops in the Confederate Army of the Potomac was approximately 22,000.
The Confederate Army of the Shenandoah was also organized
into brigades. It consisted of four brigades of three to five infantry regiments each, which totaled approximately 12,000
men. Each brigade was assigned one artillery battery. Although the combined strength of both Confederate armies was about
34,000, only about 18,000 were actually engaged at First Bull Run.
Throughout the Civil War the infantry regiment was the basic
administrative and tactical unit of armies. Regimental headquarters consisted of a colonel, lieutenant colonel, adjutant,
quartermaster, surgeon (with rank of major), two assistant surgeons, a chaplain, sergeant major, quartermaster sergeant, commissary
sergeant, hospital steward, and two principal musicians. A captain, a first lieutenant, a second lieutenant, a first sergeant,
four sergeants, eight corporals, two musicians, and one wagoner staffed each company.
The authorized strength of a Civil War infantry regiment was
about 1,000 officers and men, who were arranged in ten companies plus a headquarters and (for the first half of the war at
least) a band. Discharges for physical disability, disease, special assignments (bakers, hospital nurses, or wagoners), courts-martial,
and battle injuries all combined to reduce effective combat strength. Before too long a typical regiment might be reduced
to less than 500 troops.
Brigades were made up of two or more regiments, with four regiments
being most common. Union brigades averaged 1,000 to 1,500 infantry, while a Confederate brigade averaged 1,500 to 1,800. Union
brigades were designated by a number within their division, while Confederate
brigades were designated by the name of a current or former commander.
Divisions were formed of two or more brigades. Union divisions
contained 2,500 to 4,000 infantrymen, while the Confederate infantry division was somewhat larger, containing 5,000 to 6,000
men.
After First Bull Run, both sides eventually organized corps
by combining two or more divisions. Two or more corps constituted an army, the largest operational organization.
