The infantry regiment was the basic administrative and tactical unit of Civil War armies. Regimental headquarters consisted of a colonel, lieutenant colonel, major, adjutant, quartermaster, surgeon (with rank of major), two assistant surgeons, a chaplain, sergeant major, quartermaster sergeant, commissary sergeant, hospital steward, and two principal musicians. Each company was staffed by a captain, a first lieutenant, a second lieutenant, a first sergeant, four sergeants, eight corporals, two musicians, and one wagoner.
The authorized strength of a Civil War infantry regiment was about 1,000 officers and men, 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 infantrymen, while a Confederate brigade averaged 1,500 to 1,800. Union brigades were designated by a number within their division, and each Confederate brigade was 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.
A corps was formed of two or more divisions. Two or more corps constituted an army, the largest operational organization.
In the Eastern Theater in the fall of 1861 the principal adversaries were the Union Army of the Potomac and the Confederate Army of the Potomac (renamed the Army of Northern Virginia in June 1862). The Union Army was organized into eleven infantry divisions. In March 1862 these divisions would be reorganized into five corps of three to four divisions each. Union artillery was organized into batteries of four to six guns each, and two or more batteries assigned to each division. An artillery reserve contained eight batteries. Union cavalry was generally organized into regiments and assigned to a division. A separate cavalry command included three regiments and two additional companies. The total strength of the Union Army of
the Potomac in October 1861 was approximately 152,000 men, including a division stationed in Baltimore.
In October 1861 the Confederate Army of the Potomac consisted of two infantry corps, organized into a total of fifteen brigades. Each brigade contained from two to five regiments. Artillery was organized into a corps of approximately six batteries, while the cavalry was organized into a single brigade, with independent companies operating with some infantry brigades. The total strength of the Confederate Army of the Potomac in October 1861 was approximately 40,000 men.
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