In the Eastern Theater of the war, in September 1862 the principal adversaries were the Union Army of the Potomac and the Confederate Army of Northern Virginia. The Union army was organized into six infantry corps and on cavalry division. Artillery was organized into batteries of four to six guns each, with two or more batteries assigned to each of the army's eighteen divisions. The total strength of the Army of the Potomac during the Battle of Antietam was approximately 85,000 men.
In September 1862 the Army of Northern Virginia consisted of two infantry "commands" comprising a total of nine infantry divisions and one cavalry division. (The formation of corps was not authorized in the Confederate Army until after the Battle of Antietam.) Artillery batteries were assigned to each of the army's divisions and to the army's reserve artillery. At the time of the Battle of Antietam, the strength of the Army of Northern Virginia was about 60,000, though only about 40,000 actually participated.
For Civil War armies the infantry regiment was the basic administrative and tactical unit. Regimental headquarters consisted of a colonel, lieutenant colonel, major, adjutant, quartermaster, surgeon (a major), two assistant surgeons, a chaplain, sergeant major, quartermaster sergeant, commissary sergeant, hospital steward, and two principal musicians. Each company was generally staffed by a captain, a first lieutenant, a second lieutenant, sometimes a third lieutenant, a first sergeant, 4 sergeants, 8 corporals, 2 musicians, and 1 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 (at least for the first half of the war) a band. Discharges for physical disability, disease, special assignments (as bakers, hospital nurses, or wagoners), courts-martial, and battle injuries all combined to reduce effective combat strength. Before long a typical regiment might be reduced to fewer 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 Confederate brigades 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 Confederate infantry divisions were 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.
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