The largest operational unit during the American Civil War was referred to as an "Army" (Army of Northern Virginia; Army of the Potomac, etc.) and it was named for the department in which it operated. A total of 16 separate armies were formed by the Union
and 23 in the Confederacy.
By the summer of 1862, the armies of both sides were beginning to develop
corps headquarters to control the divisions assigned to them. In June 1862, General Lee divided his army into "wings" under
Jackson and Longstreet, respectively. These were organized formally into corps in November of that year. An informal corps
organization had existed among the Federal forces since late 1861. These were formally designated in July 1862, but first
were numbered sequentially within their respective field armies, causing some confusion. Many of the corps were still forming
at the time of the Second Manassas Campaign, hence they were not much better structured or coordinated than their Southern
counterparts. There were usually two or three divisions in each corps (sometimes more). Normally three, occasionally four,
brigades composed a division. Four or five regiments were assigned to a brigade.
Various manpower practices, plus battle losses, make it extremely difficult
to calculate strength accurately by unit designation alone. Volunteer infantry regiments were composed of ten companies, with each
authorized a maximum of 110 officers and men. A full regiment would have numbered about 1,100 officers and men. Regular
U.S. Army Infantry regiments were organized into three battalions of three companies each and Federal heavy artillery regiments
converted to infantry had three four-company battalions. As a result, they were slightly larger than the volunteer regiments.
Federal regiments at Second Manassas had an average strength of 300 men, while similar Confederate units averaged 200 men.
During the American Civil War, Northerners raised 1,696 infantry
regiments, 272 cavalry regiments, and 72 artillery regiments.
Federal artillery batteries were standardized with six M1857 12-pounder
Napoleon pieces. There were about 100 men in a full-strength Federal battery. Confederate batteries, on the other hand, ranged
from six to two pieces and usually had various models of guns. This lack of uniformity was offset by a much better higher-level
structure. Most Confederate artillery was grouped in battalions deployed under the orders of the wing commander. This arrangement
allowed greater firepower to be brought to bear more responsively than on the Federal side. There, batteries were assigned
to division, even brigade, levels, and often operated independently once the battle commenced. The Federals had 38 batteries
with 182 guns at the Battle of Second Manassas (aka Second Bull Run), while the Confederates had 184 guns grouped in 45 batteries.
The dissimilarities in centralization extended also to the cavalry.
Confederate horse was organized in a single cavalry division, responsive to the needs of the army commander. It was used primarily
to gather information. Federal practice assigned cavalry to corps and sometimes to as low as brigades. This deprived the army
commander of any reconnaissance capability of his own. There was the probability (realized in the 1862 Manassas Campaign)
that information would not get to him. This shortcoming was made worse by General Pope's philosophy of cavalry use. He caused
his subordinates to push the cavalry mercilessly in deep raids and as hard-riding escorts and headquarters guards. Rarely
was cavalry used by the Federals solely to obtain information. As a consequence, Pope frequently lost contact during critical
changes, fatally hampering his planning. Concurrently, the cavalry became so debilitated it could not be a factor when it
was needed the most or at the climax of the campaign.
