Adding to the identity crisis the XIII Corps faced in its early years was John A. McClernand's expedition against Fort Hindman at Arkansas Post.
As the Vicksburg campaign opened the XIII Corps was composed of the 9th, 10th, 11th, 12th, 13th and 14th Divisions commanded respectively by Osterhaus, Andrew Jackson Smith, Alvin P. Hovey, Leonard F. Ross and Eugene A. Carr.
In July, this division (now led by Frederick Salomon) fought at the Battle of Helena as part of the District of Eastern Arkansas under Benjamin M. Prentiss.
McClernand did not bring the full force of the corps to bear at the Battle of Champion Hill but Hovey's division led the attack on the Confederate right.
Immediately following the victory at Champion's Hill the Battle of Big Black River Bridge was again fought exclusively by the XIII Corps, Carr's division bearing the brunt of the fight.
After Vicksburg fell, William T. Sherman led an expedition back to Jackson, Mississippi to clear the city of Confederates which had gathered there.
General Carr, who temporarily left the army due to sickness, had been replaced in division command by William P. Benton.
General Banks used the XIII Corps to conduct his coastal campaign against Texas during the fall of 1863, capturing Brownsville.
This new form of the XIII Corps fought in the Battle of Fort Blakeley which led to the fall of the city of Mobile, Alabama.
In June 1865, more than two thousand Federal soldiers of the 13th Army Corps arrived in Galveston, with Major General Gordon Granger, Commanding Officer, District of Texas.
General Order No.3 informed all Texans that, "in accordance with a Proclamation from the Executive of the United States, all slaves were free."