The corps was again reconstituted for was at the Siege of Vicksburg. With so many of the Army of Tennessee's units at Vicksburg area, the garrison seemed like part of their own army and additionally nearly the entire garrison fought with the Army of Tennessee sometime or another. The Corps consisted of Forney's division, 4,500 men, Martin Smith's division with another 6,500, William Loring's division of 6,000 more, John Bowen's division of 6,000 and Carter Stevenson's division of 10,000. The Corps all in all numbered 33,000 men. The Corps fought at Champion Hill, and the siege itself. Loring and his division broke out and later on served in two other Third Corps but the rest of the 'Corps', usually known as the Army of Mississippi, surrendered on July 4.