When the American Civil War began in April 1861, there were only 16,367 men in the U.S. Army, including 1,108 commissioned officers. Approximately 20% of these officers, mostly
Southerners, resigned and joined the
 In addition, almost 200 West Point graduates who had previously left the Army, including
Bragg, would return to service at the outbreak of the war. This group's loyalties were far more sharply divided, with 92 doning Confederate gray and 102 putting on the blue of the Union Army. The U.S. Army consisted of ten regiments of
infantry, four of
artillery, two of
cavalry, two of
dragoons, and three of
mounted infantry. The regiments were scattered widely. Of the 197 companies in the army, 179 occupied 79 isolated posts in the
West, and the remaining 18 manned garrisons east of the
Mississippi River, mostly along the
Canada–United States border and on the Atlantic coast.
Southern slave states declaring secession from the Union, and with this drastic shortage of men in the army,
Abraham Lincoln called on the states to raise a force of 75,000 men for three months to put down the insurrection. Lincoln's call forced the border states to choose sides, and four seceded, making the Confederacy eleven states strong. The war proved to be longer and more extensive than anyone North or South had expected, and on July 22, 1861,
Congress authorized a volunteer army of 500,000 men.
The call for volunteers initially was easily met by patriotic Northerners,
abolitionists, and even immigrants who enlisted for a steady income and meals. Over 10,000
Germans in New York and
Pennsylvania immediately responded to Lincoln's call, and the
French were also quick to volunteer. As more men were needed, however, the number of volunteers fell and both money bounties and forced conscription had to be turned to. Nevertheless, between April 1861 and April 1865, at least two and a half million men served in the Union Army, of whom the majority were volunteers.
It is a misconception that the South held an advantage because of the large percentage of professional officers who resigned to join the
Confederate army. At the start of the war, there were 824 graduates of the
U.S. Military Academy on the active list; of these, 296 resigned or were dismissed, and 184 of those became Confederate officers. Of the approximately 900
West Point graduates who were then civilians, 400 returned to the Union Army and 99 to the Confederate. Therefore, the ratio of Union to Confederate professional officers was 642 to 283.
 (One of the resigning officers was
Robert E. Lee, who had initially been offered the assignment as commander of a field army to suppress the rebellion. Lee disapproved of secession, but refused to bear arms against his native state,
Virginia, and resigned to accept the position as commander of Virginian C.S. forces. He eventually became the commander of the
Confederate army.) The South did have the advantage of other military colleges, such as
The Citadel and
Virginia Military Institute, but they produced fewer officers. Though officers were able to resign, enlisted soldiers didn´t have this right; which meant that they usually had either to desert or to wait until their enlistment term was over in order to join the Confederate States Army. While the total number of those is unknown, only 26 enlisted men and non-commissioned officers of the regular army are known to have legally left the army to join the Confederate army when the war began.
The Union Army was composed of numerous organizations, which were generally organized geographically.
- A collection of Departments reporting to one commander (e.g.,
Military Division of the Mississippi,
Middle Military Division,
Military Division of the James). Military Divisions were similar to the more modern term
Theater; and were modeled close to, though not synonymous with, the existing theaters of war.
- An organization that covered a defined region, including responsibilities for the Federal installations therein and for the field armies within their borders. Those named for states usually referred to Southern states that had been occupied. It was more common to name departments for rivers (such as
Department of the Tennessee,
Department of the Cumberland) or regions (
Department of the Pacific,
Department of New England,
Department of the East,
Department of the West,
- A subdivision of a Department (e.g., District of Cairo, District of East Tennessee). There were also Subdistricts for smaller regions.
- The fighting force that was usually, but not always, assigned to a District or Department but could operate over wider areas. Some of the most prominent armies were:
Army of the Cumberland, the army operating primarily in
Tennessee, and later
Georgia, commanded by
William S. Rosecrans and
George Henry Thomas.
Army of Georgia, operated in the March to the Sea and the Carolinas commanded by
Henry W. Slocum.
Army of the Gulf, the army operating in the region bordering the
Gulf of Mexico, commanded by
Nathaniel P. Banks, and
Army of the James, the army operating on the
Virginia Peninsula, 1864–65, commanded by Benjamin Butler and
Army of the Mississippi, a briefly existing army operating on the Mississippi River, in two incarnations—under
John Pope and
William S. Rosecrans in 1862; under
John A. McClernand in 1863.
Army of the Ohio, the army operating primarily in
Kentucky and later Tennessee and Georgia, commanded by
Don Carlos Buell,
Ambrose E. Burnside,
John G. Foster, and
John M. Schofield.
Army of the Potomac, the principal army in the
Eastern Theater, commanded by
George B. McClellan, Ambrose E. Burnside,
Joseph Hooker, and
George G. Meade.
Army of the Shenandoah, the army operating in the
Shenandoah Valley, under
Philip Sheridan, and
Horatio G. Wright.
Army of the Tennessee, the most famous army in the
Western Theater, operating through Kentucky, Tennessee, Mississippi, Georgia, and the
Carolinas; commanded by
Ulysses S. Grant,
William T. Sherman,
James B. McPherson, and
Oliver O. Howard.
Army of Virginia, the army assembled under
John Pope for the
Northern Virginia Campaign.
Each of these armies was usually commanded by a
major general. Typically, the Department or District commander also had field command of the army of the same name, but some conflicts within the ranks occurred when this was not true, particularly when an army crossed a geographic boundary.
regular army, the permanent United States Army, was intermixed into various formations of the Union Army, forming a cadre of experienced and skilled troops. They were regarded by many as elite troops and often held in reserve during battles in case of emergencies. This force was quite small compared to the massive state-raised volunteer forces that comprised the bulk of the Union Army.
Private Samuel K. Wilson (1841–1865) of the
, Illinois Volunteer Infantry, 1862
Soldiers were organized by military specialty. The combat arms included
artillery, and other such smaller organizations such as the United States Marine Corps, which, at some times, was detached from its navy counterpart for land based operations. The
Signal Corps was created and deployed for the first time, through the leadership of
Albert J. Myer.
Below major units like armies, soldiers were organized mainly into regiments, the main fighting unit with which a soldier would march and be deployed with, commanded by a
lieutenant colonel, or possibly a
major. According to W. J. Hardee's "Rifle and Light Infantry Tactics" (1855), the primary tactics for riflemen and light infantry in use immediately prior and during the Civil War, there would typically be, within each regiment, ten
companies, each commanded by a
captain, and deployed according to the ranks of captains. Some units only possessed between four and eight companies and were generally known as
 Regiments were almost always raised within a single state, and were generally referred by number and state, e.g.
20th Maine, etc.
Regiments were usually grouped into
brigades under the command of a
brigadier general. However, brigades were changed easily as the situation demanded; the regiment was the main form of permanent grouping. Brigades were usually formed once regiments reached the battlefield, according to where the regiment might be deployed, and alongside which other regiments.
The champions of the Union
- 1861 lithograph by Currier & Ives
Several men served as generals-in-chief of the Union Army throughout its existence:
The gap from March 11 to July 23, 1862, was filled with direct control of the army by President Lincoln and
United States Secretary of War
Edwin M. Stanton, with the help of an unofficial "War Board" that was established on March 17, 1862. The board consisted of
Ethan A. Hitchcock, the chairman, with Department of War bureau chiefs
Montgomery C. Meigs,
Joseph G. Totten,
James W. Ripley, and
Joseph P. Taylor.
Scott was an elderly veteran of the
War of 1812 and the
Mexican–American War and could not perform his duties effectively. His successor,
Maj. Gen. McClellan, built and trained the massive Union
Army of the Potomac, the primary fighting force in the Eastern Theater. Although he was popular among the soldiers, McClellan was relieved from his position as general-in-chief because of his overcautious strategy and his contentious relationship with his
commander-in-chief, President Lincoln. (He remained commander of the Army of the Potomac through the
Peninsula Campaign and the
Battle of Antietam.) His replacement, Major General
Henry W. Halleck, had a successful record in the Western Theater, but was more of an administrator than a strategic planner and commander.
Ulysses S. Grant was the final commander of the Union Army. He was famous for his victories in the West when he was appointed
lieutenant general and general-in-chief of the Union Army in March 1864. Grant supervised the Army of the Potomac (which was formally led by his subordinate, Maj. Gen.
George G. Meade) in delivering the final blows to the Confederacy by engaging Confederate forces in many fierce battles in Virginia, the
Overland Campaign, conducting a war of attrition that the larger Union Army was able to survive better than its opponent. Grant laid
siege to Lee's army at
Petersburg, Virginia, and eventually captured
Richmond, the capital of the Confederacy. He developed the strategy of coordinated simultaneous thrusts against wide portions of the Confederacy, most importantly the
Carolinas Campaigns of
William Tecumseh Sherman and the
Shenandoah Valley campaign of
Philip Sheridan. These campaigns were characterized by another strategic notion of Grant's-better known as
total war—denying the enemy access to resources needed to continue the war by widespread destruction of its factories and farms along the paths of the invading Union armies.
Grant had critics who complained about the high numbers of casualties that the Union Army suffered while he was in charge, but Lincoln would not replace Grant, because, in Lincoln's words: "I cannot spare this man. He fights."
Among memorable field leaders of the army were
Nathaniel Lyon (first Union general to be killed in battle during the war),
George Henry Thomas and
William Tecumseh Sherman. Others, of lesser competence, included
Benjamin F. Butler.
The decisive victories by Grant and Sherman resulted in the surrender of the major Confederate armies. The first and most significant was on April 9, 1865, when
Robert E. Lee surrendered the
Army of Northern Virginia to Grant at
Appomattox Court House. Although there were other Confederate armies that surrendered in the following weeks, such as
Joseph E. Johnston's in
North Carolina, this date was nevertheless symbolic of the end of the bloodiest war in American history, the end of the Confederate States of America, and the beginning of the slow process of