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The Army of the Tennessee was a Union army in the Western Theater of the American Civil War, named for the Tennessee River.
It appears that the term "Army of the Tennessee" was first used within the Union Army in March 1862, to describe Union forces perhaps more properly described as the "Army of West Tennessee"; these were the troops under the command of Maj. Gen. Ulysses S. Grant in the Union's District of West Tennessee. In April 1862, Grant's troops survived a severe test in the bloody Battle of Shiloh. Then, during six months marked by discouragement and anxiety for Grant, his army first joined with two other Union armies to prosecute the relatively bloodless Siege of Corinth and then strained to hold Union positions in Tennessee and Mississippi. In October 1862, Grant's command was reconfigured and elevated to departmental status, as the Department of the Tennessee; the title of his command was thus officially aligned with that of his army. Grant commanded these forces until after his critically important victory at Vicksburg on July 4, 1863. Under other generals, starting with William Tecumseh Sherman, the army marched and fought from the Chattanooga Campaign, through the Relief of Knoxville, the Meridian Campaign, the Atlanta Campaign, the March to the Sea, the Carolinas Campaign, and to the end of the war and disbandment. This article also discusses Grant's 1861–1862 commands – the District of Southeast Missouri and the District of Cairo – because the troops Grant led in the Battle of Belmont and the Henry-Donelson campaign during that period became the nucleus of the Army of the Tennessee.
A 2005 study of the army states that it "was present at most of the great battles that became turning points of the war—Fort Donelson, Vicksburg, and Atlanta" and "won the decisive battles in the decisive theater of the war." More poetically, in 1867, apparently speaking of the Atlanta campaign, General Sherman said that the Army of the Tennessee was "never checked—always victorious; so rapid in motion—so eager to strike; it deserved its name of the 'Whip-lash,' swung from one flank to the other, as danger called, night or day, sunshine or storm."
History remembers the Army of the Tennessee as one of the most important Union armies during the Civil War, an army intimately associated with the Union's two most celebrated generals, Ulysses S. Grant and William Tecumseh Sherman. It is thus rather ironic that frequent military reorganizations and looseness of usage during the war itself make it difficult to pinpoint the exact date at which this army formally came into existence. It should suffice to note that the "nucleus [of troops] around which was to gather the... Army of the Tennessee" first took shape in 1861–1862, while Grant was headquartered at Cairo, Illinois. Those troops continued under Grant in his next command, the distinct District of West Tennessee; they were then sometimes, and perhaps most appropriately, called the "Army of West Tennessee." However, army correspondence began using the term "Army of the Tennessee" in March 1862; that term soon became commonplace and naturally lived on when Grant's command was elevated to departmental status in October 1862, as the Department of the Tennessee. During the course of the war, elements of the Army of the Tennessee performed many tasks, and the army evolved with the addition and subtraction of many units. It is not feasible to chronicle every such development here, even at the corps level. Rather, this article traces the main thrust of the army's development and its most memorable activities. At any given time, substantial numbers of troops were engaged in activities not discussed here. For example, in April 1863, less than half of Grant's departmental strength was directly engaged in the Vicksburg Campaign.