During the siege of Vicksburg, Confederate General Joseph E. Johnston had gathered a force of 30,000 men in Jackson, Mississippi, with the intention of relieving the garrison under the command of John C. Pemberton that was trapped inside Vicksburg. After Pemberton surrendered to Grant on July 4, Johnston advanced towards the rear of Grant's forces. In response to this threat, Grant instructed Sherman to attack Johnston. Sherman conducted the ensuing Jackson Expedition, which concluded successfully on July 25 with the re-capture of the city of Jackson. This helped ensure that the Mississippi River would remain in Union hands for the remainder of the war. According to military historian Brian Holden-Reid, Sherman finally "had cut his teeth as an army commander" with the Jackson Expedition.