On November 9, 1861, the United States War Department issued General Orders, No. 97, which divided the Western Department into the Department of New Mexico, the Department of the Ohio, the Department of Western Virginia, and the Department of the Missouri. The directive appointed Major General Henry Halleck as commander of the latter.
In February 1862, Halleck organized the Union troops operating in the State of Missouri, creating the Army of the Mississippi. Halleck named Brigadier-General John Pope as commander of the new army. Pope assumed command on February 23 and established his headquarters at Commerce, Missouri. Initially, the small army comprised only two divisions.
In March, the size of the Army of the Mississippi swelled to nearly 25,000 soldiers, to prepare for Pope’s advance on the Mississippi River town of New Madrid, Missouri. At its largest extent, the army comprised five infantry divisions, a flotilla brigade, and several brigades of cavalry. The infantry division commanders were Brigadier-Generals David S. Stanley, Schuyler Hamilton, John M. Palmer, Eleazer A. Paine, and Joseph B. Plummer. Brigadier-General Gordon Granger commanded the cavalry division of two regiments, and Colonel Napoleon B. Buford led the flotilla brigade.
Union Victories at New Madrid and Island No. 10
On March 14, 1862, the Army of the Mississippi forced the withdrawal of Confederate troops from New Madrid to Island No. 10 on the Mississippi River. Then, with the help of Flag Officer Andrew Hull Foote’s naval forces, Pope subdued the Confederate garrison on the island on April 7, 1862, taking nearly 7,000 prisoners, while suffering fewer than one hundred casualties. Pope’s victory gave the Union control of the river as far south as Memphis, Tennessee.
Siege of Corinth
On April 17, 1862, Pope’s command sailed to Hamburg Landing on the Tennessee River and joined the Union advance upon Corinth, Mississippi. The Army of the Mississippi took part in the Siege of Corinth from April 29 to May 30, 1862, serving on the extreme left of the Union army.
In June 1862, President Lincoln summoned Pope east to take command of the newly created Army of Virginia. On June 26 Brigadier-General William S. Rosecrans succeeded Pope as commander of the Army of the Mississippi.
Lincoln Promotes Halleck
On July 11, 1862, President Abraham Lincoln telegraphed Halleck that he was being promoted to General-in-Chief of the Union Army, effective July 23. Ulysses S. Grant became the de facto commander of the Department of the Missouri because the army assigned no one to the position, and Grant made his reports directly to Halleck.
Iuka and Corinth
During that period, Grant detached the 2nd and 6th Divisions of the Army of West Tennessee to join the Army of the Mississippi garrisoned at Corinth. Bolstered by the two additional divisions, the Army of the Mississippi took part in the Union victory at Battle of Iuka (September 19, 1862). Afterward, the combined force returned to Corinth, where it successfully defended the town from a Confederate attack at the Second Battle of Corinth on October 3 and 4, 1862.
Shortly after the Second Battle of Corinth, the War Department issued General Orders No. 159, on October 16, 1862, placing Ulysses S. Grant in command of the newly created Department of the Tennessee. Grant took command on October 25. The Army of the Mississippi ceased to exist the next day, when Grant issued General Orders No. 2, Department of the Tennessee, stating, “The army heretofore known as the ‘Army of the Mississippi,’ being now divided and in different Departments, will be discontinued as a separate Army.”