 Complete the following phrase:  In June, 1863, McNairs Brigade was reassigned to Walkers later Frenchs 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 General Pembertons besieged army at Vicksburg. Johnston had been gathering troops at Jackson, intending to relieve pressure on Lt. Gen. John C. Pembertons beleaguered garrison. Johnston cautiously advanced his 30,000 soldiers toward the rear of Grants army surrounding Vicksburg. In response, Grant ordered Sherman to deal with Johnstons threat. By July 1, 1863, Johnstons 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 