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Second Manassas: Order of Battle

The basic unit of military organization in the 1860's was the company, comprised of 100 officers and enlisted men. A total of ten companies formed a regiment; five regiments a brigade; three brigades a division; and three divisions a corps. Casualties, desertions, expired enlistments, and other factors frequently caused drastic reductions in the sizes of all organizations. Often, for example, a division fielded fewer men than a single regiment at full strength.

The largest operational unit, armies, were named for the department in which they operated. A total of 16 separate armies were formed by the Union, and 23 in the Confederacy.

Nearly 3,000,000 men served in Union armies during the Civil War. Northerners raised 1,696 infantry regiments, 272 cavalry regiments, and 72 artillery regiments. Definitive statistics for Confederate armies are lacking, but estimates place the total number of men under arms between 600,000 and 900,000.