During the American Civil War, the North generally named a battle after the closest river, stream or creek, and the South tended to name battles after towns or railroad junctions. Hence the Confederate name Manassas after Manassas Junction, and the Union name Bull Run for the stream Bull Run.

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.