Sorry for any confusion, but I am referring to the battle of Jackson, on May 14, 1863. The stubborn fight put up by Gregg two days prior at the battle of Raymond convinced Grant he needed to deal with Johnston and destroy Jackson before turning his attention back to Pemberton and his Vicksburg army. Had Johnston defended Jackson and held the rail center, Confederate reinforcements would have been able to arrive, disembark and quickly reinforce Johnston's growing "Army of Relief." By abandoning Jackson, he made it impossible to quickly gather his reinforcements into a threatening strike force to the east of Grant, allowing Grant's forces to then turn and deal with Pemberton at Champion Hill on May 16.I might not be following. How exactly does Johnston relieve the siege of Vicksburg by standing and waiting for Grant to attack him?