After Jackson was captured, the forces of Johnston and Pemberton were cut off from each other. On May 16, Grant's men defeated Pemberton decisively at the Battle of Champion Hill. By May 18, the Union soldiers had reached Vicksburg and placed the city under siege. The Siege of Vicksburg continued until July 4, when Pemberton surrendered. During the siege, reinforcements from across the Confederacy continued to be diverted to Johnston, who eventually amassed 32,000 men. Named the Army of Relief, Johnston's force did not move against Grant until July 1, and then upon reaching the Union lines at the Big Black River two days later, decided that the defenses could not be taken and did not bring on a battle. Johnston ordered a retreat on the 5th, and on July 7, Johnston's retreating troops reoccupied Jackson. Grant responded by sending Sherman with 46,000 men to follow Johnston. This movement, known as the Jackson Expedition, reached the city of July 10. The city was soon placed under siege; a limited Union attack that mistakenly occurred was repulsed on July 12. Johnston again abandoned Jackson on the night of July 16/17.