During the Civil War, Union forces in Tennessee were part of several different federal armies, primarily the Army of the Cumberland, the Army of the Ohio, and the Army of the Tennessee. An army from the early Department of the Missouri under the command of General U. S. Grant captured Forts Henry and Donelson in February 1862. By the time of the battle of Shiloh that April, however, most of this force under Grant’s command was known as the Army of the Tennessee.
The Army of the Ohio, once part of the Department of the Ohio under the command of General Don Carlos Buell, also participated at Shiloh. From this force, after the battle of Perryville, Kentucky, in the fall of 1862, would emerge the Army of the Cumberland. In late October, command of the army was taken away from Buell and given to General William S. Rosecrans. Upon his arrival in Nashville in November 1862, Rosecrans brought news that the Union forces gathered there would now be known as the Army of the Cumberland. The Army of the Ohio in name would resurface in 1863 as the army under the command of General Ambrose Burnside in East Tennessee. This force won the battle of Knoxville and successfully defended the upper East Tennessee region for the remainder of the war.
From November 1862 to November 1863, the Army of the Cumberland would be the primary Union force in the state, participating in the battles of Stones River and Chattanooga in Tennessee and the battle of Chickamauga in Georgia and conducting the brilliant Tullahoma Campaign of 1863. After the loss at Chickamauga in September 1863, however, command of the Army of the Cumberland passed from Rosecrans to Major General George H. Thomas, whose nickname was the “Rock of Chickamauga.” Thomas’s army joined forces with the Army of the Tennessee, led by William T. Sherman, during the Chattanooga battles of November 1863, a successful campaign under the overall leadership of Grant. Thomas’s forces also fought in the Atlanta campaign of 1864 as part of Sherman’s massive invasion force.
When Confederate General John Bell Hood launched his Tennessee invasion in the fall of 1864, Sherman detached corps from the old Army of the Cumberland and from the Army of the Tennessee to buttress Thomas’s army in Nashville. These forces successfully defended the Union occupation of Nashville at the battles of Spring Hill, Franklin, and Nashville in late 1864. Thomas, whose reputation as a field commander is indelibly linked with the name of the Army of the Cumberland, led the final devastating Union assault at Nashville, an attack which all but destroyed the Confederate Army of Tennessee.
The Union armies in Tennessee were known for their several excellent generals, especially Grant, Sherman, Thomas, John A. McClernand, Charles F. Smith, and Philip M. Sheridan, who not only played a significant role in Tennessee but in Georgia, Mississippi, and Virginia and in other U.S. military campaigns after the war.