On Friday, October 16, 1863 the Union War Department created the Military Division of the Mississippi, combining the former Departments of the Ohio, Cumberland, and the Tennessee, under the command of General Ullyses Grant, the conqueror of Vicksburg, Mississippi.
Grant was given command from the Mississippi River east to the mountains. On the following day, Grant, in route to Louisville, Kentucky, accidentally met Secretary of War Edwin Stanton, who was journeying to Louisville to see Grant. Stanton gave Grant word of his new command and allowed him to appoint his subordinate commanders.
Grant acted immediately replacing William Rosecrans at Chattanooga with General George Thomas. General William Tecumseh Sherman was given command in the Department of Tennessee, and Ambrose Burnside was retained to command in the Department of Ohio.