This article is designed to give background into the organization and tactics of Civil War armies. This brief survey is by no means exhaustive, but it should give enough material for a better understanding of the capabilities of the forces that fought the American Civil War. Understanding these capabilities should give insight into the reasoning behind the decisions made by commanders on both sides.King-Robertson-Clay, p. 1.
Organization
The US Army in 1861
The United States Army, known also as the Regular Army, was a small, scattered constabulary force ill-equipped to fight a major conflict on the eve of the Civil War. Partly this was due to a Jeffersonian ideal which saw a standing army as a threat to democracy and whose elitist, hierarchical structure was in conflict with the concepts of equality and rugged individualism. It was also due to a realistic assessment of the situation: since the War of 1812, the threat of external invasion was unlikely, and a small force was sufficient for conquering and displacing Native American tribes. If the need arose, the Regular Army could be supplemented by raising United States Volunteers and incorporating state militia.Newell & Shrader, p. 1-3
As of January 1, 1861, the Regular Army on paper consisted of 16,403 officers and enlisted men, but only 14,658 were actually present for duty. A further 340 civilian clerks, teamsters and laborers were also employed. The Army was split into two organizations: the Line of the Army, which constituted the actual fighting force in the field, and the Staff Departments, which provided administrative and logistical support. The Secretary of War had control over the Army but by tradition would appoint a "major general commanding in chief" (or general-in-chief) to oversee field operations. The authority of the general-in-chief was unclear however as it was an extralegal position rather than one created by Congressional statue. At the time it was held by Lieutenant General Winfield Scott, the 75‑year‑old hero of the Mexican–American War.Newell & Shrader, p. 3-8King-Robertson-Clay, p. 1-2
The Line of the Army consisted of nineteen regiments: ten infantry regiments, five mounted regiments (two cavalry, two dragoon, and one of mounted riflemen), and four artillery regiments. Each infantry and mounted regiment was divided into ten companies while the artillery each consisted of twelve, for a total of 197 companies. However, all of the infantry and cavalry companies, along with 30 of the artillery companies, were west of the Mississippi River, scattered among 79 posts. All field units fell within one of six geographical departments and answered to its commander, who in turn reported directly to the general-in-chief.
The Staff Departments or bureaus provided the necessary administrative support to keep the army fed, clothed and armed, reflecting the modern concepts of combat support and combat service support. Modeled after the British system, each bureau operated independently of the other in setting their own standards and operating procedures. The head of each staff department, sometimes called the bureau chief, reported directly to the Secretary of War but also tended to have a direct relationship with Congress too. In total there were 352 staff officers and 831 enlisted personnel in the Staff Departments at the start of the year, suitable for the time but their numbers would necessarily grow to meet the needs of the expanding army.
The Regular Army remained under Federal control during the war, although 270 officers and hundreds of enlisted men joined with the South. The question of what to do with it once hostilities started however was hotly debated. One option would have been to use the Regulars as a training and leadership cadre for the newly-formed volunteer units. The government ultimately decided to keep the Regulars together, to form a core of reliable reserves for the upcoming battles. Although it was expanded in size to a peak authorized strength of 41,000, there were never more than 26,000 men serving in the Regular Army at any one time. Service in the Volunteers was preferable to the average soldier as discipline was less severe, the length of service was not as long, and their friends could all join and fight together. By 1864 many Regular units were so depleted they were withdrawn from the frontlines.Newell & Shrader, p. 39-43King-Robertson-Clay, pp. 2–3.
Raising armies
Just days after the fall of Fort Sumter, President Lincoln declared the Confederate states in open rebellion. Using the authority granted to him under the 1795 federal militia statue, he called for the loyal states to furnish 75,000 militia for three months (the statutory limit). Two weeks later he issued a second call, this time asking for 42,000 state volunteers and the expansion of the regular army (the Regular Army remained under the control of the Federal government, although 270 officers and hundreds of enlisted men would leave the ranks to join the Confederates). Both calls were met with a vast outpouring of volunteers. Although nothing under the 1795 statue authorized the follow-up call, Lincoln justified it on the basis of the "war power of the government" and Congress retroactively approved when it was back in session.Guelzo (2012), Ch 4 - To War upon Slavery: The East and Emancipation, 1861-1862Axelrod (2017), Ch 6 - The North Mobilizes
Lincoln hoped this show of force would be enough to dissuade the rebels, but in fact it had the opposite effect. States like Virginia had wanted to sit out the conflict, but when forced to choose they sided with the Confederates as pro-secession feelings became inflamed. The Confederate military mirrored that of the Federal, with a small professional army on paper but a much larger Provisional Army of the Confederate States under which regiments of state volunteers were formed. In March the Confederate Congress had authorized the acceptance of 100,000 one-year volunteers. It was met with such overwhelming response that recruiters had to turn away volunteers for lack of arms and equipment. This did not stop a second call for 400,000 volunteers in May, with terms of three years or until the war's end.Axelrod (2017), Ch 5 - The South Mobilizes
As the states bore the responsibility of recruiting and organizing these volunteer units, there was no standard procedure in place, and so it became a matter of local political initiative. A community held a recruitment rally where the local young men were encouraged to join up. Organizing themselves into companies, these recruits were moved to a "camp of rendezvous" which might be a local park or fairground. Here the companies would be organized into regiments and issued arms, uniforms and equipment, assuming any were available. Either here or, if it was too small, after moving on to a "camp of instruction" the officers were chosen and the men taught the basics of military drill and discipline.Guelzo (2012), Ch 6 - The Soldier's Tale
Although neither side faced recruitment difficulties at first, as the war progressed and casualties mounted it became increasingly harder to refill the ranks. The first conscription laws in American history were passed by the Confederate government in April 1862, followed by the Federal government in March 1863. Neither law was popular with the common folk: those with money could afford to hire a substitute to take their place or pay commutation fees, and in the South slaveowners with more than twenty slaves were exempt. The mere threat of being conscripted was encouragement enough to convince many men to volunteer willingly, and so relatively few were actually conscripted.
The North in particular tried various means to limit the effect of conscription on the populace. Bounties were paid out to those willing to join voluntarily, and various reenlistment bonuses were offered to keep veterans within the ranks. The Invalid Corps was created so that wounded soldiers filled out essential rear area duties, freeing the able-bodied for frontline duty. Perhaps the most radical measure however was enlisting African Americans for service. The Militia Act of 1862 gave Lincoln the authority to do so, but it was only after the Emancipation Proclamation was any effort made. By May 1863 the Bureau of Colored Troops had been created to oversee the organization of Black soldiers as United States Colored Troops. In total, 178,000 African Americans fought in the Union army and another 10,000 in the navy.
Ultimately, both sides succeeded in raising armies the size of which had never been seen before in the nation's history. A total of 2,128,948 men would serve the Union cause throughout the course of the war, representing a third of the North's military-age male population. Confederate records were either lost or destroyed during the war, but it is estimated that a little under one million men served, representing fourth-fifths of the South's military-age male population.
Unit Structure
The unit structure into which the expanding armies were organized was generally the same for Federals and Confederates, reflecting the common roots of both armies. The Federals began the war with a Regular Army organized into an essentially Napoleonic, musket-equipped structure. Both sides used a variant of the old Regular Army structure for newly formed volunteer regiments. The Federal War Department established a volunteer infantry regimental organization with a strength that could range from 866 to 1,046 (varying in authorized strength by up to 180 infantry privates). The Confederate Congress field its 10‑company infantry regiment at 1,045 men. Combat strength in battle, however, was always much lower (especially by the time of the Overland Campaign) because of casualties, sickness, leaves, details, desertions, and straggling.
The battery remained the basic artillery unit, although battalion and larger formal groupings of artillery emerged later in the war in the eastern theater. Four under strength Regular artillery regiments existed in the US Army at the start of the war and one Regular regiment was added in 1861, for a total of 60 batteries. Nevertheless, most batteries were volunteer organizations. For the first years of the war and part way into the Overland Campaign, a Federal battery usually consisted of six guns and had an authorized strength of 80 to 156 men. A battery of six 12‑pound Napoleons could include 130 horses. If organized as "horse" or flying artillery, cannoneers were provided individual mounts, and more horses than men could be assigned to the battery. After the battle of Spotsylvania in 1864, most of the Army of the Potomac's artillery was reorganized into four-gun batteries. Their Confederate counterparts, plagued by limited ordnance and available manpower, usually operated throughout the war with a four-gun battery, often with guns of mixed types and calibers. Confederate batteries seldom reached their initially authorized manning level of 80 soldiers.
Prewar Federal mounted units were organized into five Regular regiments (two dragoon, two cavalry, and one mounted rifle), and one Regular cavalry regiment was added in May 1861. Although the term "troop" was officially introduced in 1862, most cavalrymen continued to use the more familiar term "company" to describe their units throughout the war. The Federals grouped two companies or troops into squadrons, with four to six squadrons comprising a regiment. Confederate cavalry units, organized in the prewar model, were authorized 10 76-man companies per regiment.King-Robertson-Clay, pp. 3–4. Some volunteer cavalry units on both sides also formed into smaller cavalry battalions. Later in the war, both sides began to merge their cavalry regiments and brigades into division and corps organizations.