On October 3, 1863, Smith was assigned to duty as chief engineer of the Army of the Cumberland (and a couple of weeks later, the Military Division of the Mississippi). As such he conducted the engineer operations and launched the Battle of Brown's Ferry, which opened the "Cracker Line" to provide supplies and reinforcements to the besieged troops in Chattanooga. Of this action the House Committee on Military Affairs reported in 1865 that "as a subordinate, General WF Smith had saved the Army of the Cumberland from capture, and afterwards directed it to victory." Smith was now again nominated for the rank of major general of volunteers, and Ulysses S. Grant, who was much impressed with Smith's work, insisted strongly that the nomination should be confirmed, which was accordingly done by the Senate on March 9, 1864. Grant, according to his own statement "was not long in finding out that the objections to Smith's promotion were well grounded," but he never said what the grounds were, while Smith, contributing to a major series of war memoirs, maintained that they were purely of a personal character.