As if by magic the hundreds of thousands of volunteers were armed with the munitions of War and marched to the battle-front. The great Lincoln, under the constitutional provisions, was commander-in-chief of the citizen armies, and worked in conjunction with his War Department at Washington. The military genius of a trained fighter was needed and from the outbreak of the War until November 6, 1861, Brevet-Lieutenant Winfield Scott was in command; then came Major-General George B. McClellan, a man of great caution, until March 11, 1862. From that time until July 12, 1862, the Government was without a general commander until Major-General Henry W. Halleck took control and continued till March 12, 1864. It was then that Lieutenant-General Ulysses S. Grant was called upon to end the struggle. Under these military leaders the great fighting force of volunteers was organized into armies. The first of these patriot legions was the Army of the Potomac.
Army of the Potomac was called into existence in July, 1861, and was organized by Major-General George B. McClellan, its first commander; November 5, 1862, Major-General A. E. Burnside took command of it; January 25, 1863, Major-General Joe Hooker was placed in command, and June 27, 1863, Major-General George G. Meade succeeded him.
Army of Virginia was organized August 12, 1862. The forces under Major-Generals Fremont, Banks, and McDowell, including the troops then under Brigadier-General Sturgis at Washington, were consolidated under the command of Major-General John Pope; and in the first part of September, 1862, the troops forming this army were transferred to other organizations, and the army as such discontinued.
Army of the Ohio became a power, November 9, 1861. General Don Carlos Buell assumed command of the Department of the Ohio. The troops serving in this department were organized by him as the Army of the Ohio, General Buell remaining in command until October 30, 1862, when he was succeeded by General W. S. Rosecranz. This Army of the Ohio became, at the same time, the Army of the Cumberland. A new Department of the Ohio having been created, Major-General H. G. Wright was assigned to the command thereof; he was succeeded by Major-General Burnside, who was relieved by Major-General J. G. Foster of the command of the Department and Army. Major-General J. M. Schofield took command January 28, 1864, and January 17, 1865, the Department was merged into the Department of the Cumberland.
Army of the Cumberland developed from the Army of the Ohio, commanded by General Don Carlos Buell, October 24, 1862, and was placed under the command of Major-General W. S. Rosecranz; it was also organized at the same time as the Fourteenth Corps. In January, 1863, it was divided into three corps, the Fourteenth, Twentieth and Twenty-first; in September, 1863, the Twentieth and Twenty-first Corps were consolidated into the Fourth Corps. October, 1863, General George H. Thomas took command of the army, and the Eleventh and Twelfth Corps were added to it. In January, 1864, the Eleventh and Twelfth Corps were consolidated and known as the Twentieth Corps.
Army of the Tennessee was originally the Army of the District of Western Tennessee, fighting as such at Shiloh, Tennessee. It became the Army of the Tennessee upon the concentration of troops at Pittsburg Landing, under General Halleck; and when the Department of the Tennessee was formed, October 16, 1862, the troops serving therein were placed under the command of Major-General U. S. Grant. October 24, 1862, the troops in this Department were organized as the Thirteenth Corps; December 18, 1862, they were divided into the Thirteenth, Fifteenth, Sixteenth and Seventeenth Corps. October 27, 1863, Major-General William T. Sherman was appointed to the command of this army; March 12, 1864, Major-General J. B. McPherson succeeded him; July 30, 1864, McPherson having been killed, Major-General O. O. Howard was placed in command, and May 19, 1865, Major-General John A. Logan succeeded him.
Army of the Mississippi began operations on the Mississippi River in Spring, 1862; before Corinth, Mississippi, in May, 1862; Iuka and Corinth, Mississippi, in September and October, 1862.
Army of the Gulf operated at Siege of Port Hudson, Louisiana, May, June, and July, 1863.
Army of the James consisted of the Tenth and Eighteenth Corps and Cavalry, Major-General Butler commanding and operating in conjunction with Army of the Potomac.
Army of West Virginia was active at Cloyd's Mountain, May 9 and 10, 1864.