In November 1861, Union Major General George B. McClellan was called upon to lend his prestige and organizational talents to the task of commanding the Army of the Potomac, the grand army raised to suppress the Southern rebellion. Over the next few months he organized and trained that army and molded it into the form it would maintain until war’s end in 1865. Ninety-day volunteers were replaced with three-year men, regular units were added as they came East, and brigades and divisions with their own artillery were organized, as were cavalry units. Technical services such as medical, signal and quartermaster units also began to function, and the whole was trained and exercised while defending Washington, D.C. Men from all the states, as well as the farflung Regulars, concentrated around the capital, drilling in larger and larger units.
The Regulars needed the drill as much as the volunteers–they had been scattered in company-size garrisons for years, chasing Indians. Only one post in the whole United States was garrisoned by all three arms–infantry, artillery and cavalry–prior to the Civil War, and the Regulars had not faced a European-style enemy since 1848.
The Regulars’ evolution mirrored that of the volunteers. The first battalion of infantry and cavalry to come East became a Provost Brigade, separated into a nine-regiment Infantry Reserve Brigade and a three-regiment Cavalry Reserve Brigade. The artillery went into a multiple-brigade Artillery Reserve. By early 1862 the 1st and 2nd brigades, Reserve Division, contained nine exclusively Regular regiments, while the cavalry reserve contained three regiments of U.S. Cavalry. Each division in the Army had a Regular battery brigaded with two or three volunteer batteries, and the Artillery Reserve contained a Regular Horse Brigade (four batteries), a Regular Light Brigade (six batteries), and a fifth Volunteer Brigade with two U.S. batteries. The designation ‘Reserve marked the Regulars as the dependable backbone of the Army.
