When the Civil War began, the Regular Army of the United states numbered 16,367 officers and men comprising 198 line companies distributed among 4 artillery regiments, 5 mounted regiments, and 10 regiments of infantry. The army also embraced 8 staff bureaus-the Adjutant General's, Inspector General's, Judge Advocate General's, Quartermaster General's Subsistence, Medical, Pay, and Ordnance departments-a Corps of Engineers, and a Corps of Topographical Engineers. Dozens of Regular officers resigned their commissions in 1861 to serve with the Confederacy, although few enlisted men followed their lead. In stead of using the remainder as a nucleus of professionalism withing the volunteer forces of the North, the government kept the Regular Army a separate service, and Regulars were parceled out to various armies according to need. Numerous officers and men, however, were granted leave from their units to accept higher rank in the volunteers. At war's end, each returned to his original rank and unit, more than one major general of volunteers reverted to a captaincy in the regular establishment. Various administrative changes took place in the Regulars during the conflict. Two that occurred in 1863 affected the staff departments: the Corps of Topographical Engineers and the Corps of Engineers, previously separate entities, joined under a common organization; and the Signal Corps was established as a permanent bureau. Other organizational changes affected the line arms. In May 1861 a sixth mounted regiment was formed and 3 months later a reorganization resulted in the 1st U.S. dragoons being redesignated the 1st U.S. Cavalry, the 2nd Dragoons becoming the 4th and 5th Cavalry respectively (to the detriment of unit morale), and the newest regiment being christened the 6th U.S. Cavalry. Also in May 1861, a fifth artillery regiment was authorized and was the first to be composed exclusively of light field batteries. That same month, 9 new infantry regiments were ordered recruited. Unlike the older outfits, composed of 10 companies three 8-company battalions. The Regulars served in every theater of operations. The artillery regiments were broken up, each sending some companies to serve in the East, others in the West. Centralized administration suffered because, as one artillery officer lamented, "regimental organization simply went to pieces." Of the cavalry regiments, the 1st, 2nd, 5th, and 6th served prominently in the Army of the Potomac. Most heavily engaged were the 5th and 6th, a battalion of the former losing all but one of its officers during a dramatic charge against Confederate infantry and artillery at Gaines' Mill and the latter participating in 57 engagements from Williamsburg to Appomattox. The 3rd Cavalry served entirely in the West, as did most of the 4th Cavalry. A mainstay of the Army of the Cumberland, the latter for a time lacked 2 companies, which formed a headquarters escort unit in the Army of the Potomac. Among the infantry regiments, the 1st and 2nd were broken up, serving in both the east and the West. The 3rd, 4th, 6th, 7th, 8th, 10th, 11th, 12th, 14th, and 17th infantry saw action mostly in the Army of the Potomac. The 5th infantry remained in the southwest and 9th in California throughout the conflict, while the 13th, 15th, 16th, 18th, and 19th regiments fought in the Western armies. Portions of the latter 4 outfits formed the Regular brigade of the West, a counterpart to the brigade that included most of the Regular infantry serving in the Army of the Potomac. As with the Confederate army, the Union army had to rely heavily on "Volunteers" from the several states. The state "volunteers" elected their own officers and initially furnished their own weapons and uniforms and as in the Confederate forces bore the brunt of the fighting.

Source: Mostly taken from "The Historical Times Encyclopedia of the Civil War."