In June, 1863, McNair's Brigade was reassigned to William H.T. Walker's (later French's) Division of the Army of the Department of Mississippi and Eastern Louisiana, under the overall command of General Joseph E. Johnston who was assigned the mission of organizing a force to attempt to relieve Lieutenant General John C. Pemberton's besieged army at Vicksburg. Johnston had been gathering troops at Jackson, intending to relieve pressure on Pemberton's beleaguered garrison. Johnston cautiously advanced his 30,000 soldiers toward the rear of Grant's army surrounding Vicksburg. In response, Grant ordered Sherman to deal with Johnston's threat. By July 1, 1863, Johnston's force was in position along the Big Black River. Sherman used the newly arrived IX Corps to counter this threat. On July 5, the day after the surrender of Vicksburg was made official; Sherman was free to move against Johnston. Johnston hastily withdrew his force across the Big Black River and Champion's Hill battlefields with Sherman in pursuit. Sherman had with him the IX Corps, XV Corps, XIII Corps, and a detachment of the XVI Corps. On July 10 the Union Army had taken up position around Jackson. The heaviest fighting in the Siege of Jackson came on July 11 during an unsuccessful Union attack, which resulted in heavy casualties. Instead of risking entrapment, Johnston chose to evacuate the state capital and withdrew on July 16. Sherman's forces occupied the city the following day.