Army of Mississippi or Army of the Mississippi is correct. I was changed later.
Army of Mississippi (March 1862)
This army, at times known by the names Army of the West or Army of the Mississippi (the latter particularly at Shiloh), was one of the most important in the Western theater, with battles from Shiloh to Perryville. It was organized on March 5, 1862, and portions of the Army of Pensacola were added on March 13. It was consolidated with the Army of Central Kentucky and the Army of Louisiana on March 29. On November 20, 1862, it was renamed the Army of Tennessee.
Command History
Gen. P.G.T. Beauregard: March 5, 1862 (assumed March 17, 1862) to March 29, 1862
Gen. Albert Sidney Johnston: March 29, 1862 to April 6, 1862; Shiloh
Gen. P.G.T. Beauregard: April 6, 1862 to May 6, 1862
Gen. Braxton Bragg: May 6, 1862 (assumed May 7, 1862) to July 5, 1862
Maj. Gen. William J. Hardee (temp): July 5, 1862 to August 15, 1862
Gen. Braxton Bragg: August 15, 1862 to September 28, 1862
Lt. Gen. Leonidas Polk (temp): September 28, 1862 to November 7, 1862; Perryville
Gen. Braxton Bragg: November 7, 1862 to November 20, 1862