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 14 uniformity within its field artillery batteries by using 3-inch or 12-pounder "Napoleon" guns.

Although standardization of weapons eased some field artillery problems, others remained. With the organization of army corps, half of each corps' divisional artillery was withdrawn to form an Army of the Potomac artillery reserve of three artillery brigades. The army corps had no artillery except that in its divisions, and division commanders were loath to lose control over their guns. By 1863 most commanders recognized that their heavy losses stemmed from frontal infantry assaults on entrenched positions. Rifles, with a maximum range of a thousand yards and an effective range of half that distance in the hands of good marksmen, plus supporting artillery fire presented formidable obstacles to mass attacks. Field artillery could break up the infantry's defensive positions, but with the guns scattered among the corps' divisions the coordination and concentration of artillery fire was difficult to orchestrate.

After the Union defeat at Chancellorsville in 1863 the field artillery batteries were withdrawn from the divisions of the Army of the Potomac and concentrated in field artillery brigades directly under the corps headquarters. These brigades, with four to six batteries and their own officers and staff, rendered efficient service, which resulted in a reduction in the amount of artillery needed and in the dependence on the army's artillery reserve without any reduction in fire support. The reserve gradually became a place for recuperation and reorganization of battered batteries.

To be successful, armies in the Civil War needed more than infantry, field artillery, and cavalry. As the Regular Army had few engineer units, volunteer infantry regiments often served as engineers. In the Army of the Potomac these regiments eventually formed a brigade. Army corps in other Union armies also employed infantry as engineers. As the medical, ordnance, signal, and quartermaster department lacked units, their field tasks were performed by personnel assigned to the departments or by soldiers detailed from the line regiments or by hired civilians.

To coordinate all the activities within an army corps, supporting staffs grew at each level—corps, division, brigade, and regiment. Eventually an adjutant, commissaries of muster and subsistence, and a chief of ordnance comprised the divisional staff. The staff for the brigade included an adjutant, quartermaster, commissary of subsistence, and, when the brigade served as an independent command, an ordnance officer. In addition, general officers continued to have aides as their personal staff.

As army corps evolved, the method by which they were designated changed. Initially each Union army corps was designated within an army—i.e., I Army Corps, Army of the Potomac—but later the practice of numbering them consecutively without any reference to the army to which they were assigned was adopted. Divisions within army corps were numbered consecutively, as were brigades within divisions.