Immediately after the surrender of Vicksburg, Gen. Ulysses Grant sent a large force east to pursue Confederate General Joseph Johnston. Johnston, who, with a large force of up to 30,000 Rebels, had been lurking east of Vicksburg for many weeks during the Union siege. Once Johnston heard that Vicksburg had surrendered, he knew Grant would come after him with his large army, most of which were now free from siege duties to attack. Johnston rapidly withdrew his force east toward Jackson, Mississippi, the capitol city of the state.