The Union army seals the fate of Vicksburg by defeating the Confederates at the Battle of Champion Hill, Mississippi. Union General Ulysses S. Grant had successfully run the Confederate gauntlet at Vicksburg and placed the Army of the Tennessee south of the stronghold, the Rebels' last significant holding on the Mississippi River. But he did not move directly on Vicksburg because he knew Joseph Johnston was assembling a Confederate force in Jackson, Mississippi, 40 miles east of Vicksburg.
Instead, Grant advanced toward Jackson and prevented Johnston from uniting with the Vicksburg garrison, headed by John C. Pemberton. After boldly attacking and defeating the Confederates at Jackson, Grant left William T. Sherman's corps to hold Johnston at bay. The Confederates were divided not only by Grant's army, but also by conflicting strategy. Johnston wanted Pemberton to head into northern Mississippi to join forces with his own army. But Pemberton insisted on sticking close to Vicksburg and defending the city.
Grant sent his other two corps, commanded by James McPherson and John McClernand, to take on Pemberton. They found the Confederates on Champion Hill, about halfway between Jackson and Vicksburg. There, some 30,000 Union troops attacked 20,000 Confederates. The battle swayed back and forth, but the Federals eventually gained the upper hand. Pemberton's men were forced to retreat, and one division was completely cut off from the rest of the army.
Although McClernand's timidity kept the rout from being complete, the engagement was still the decisive action of the Vicksburg campaign. Pemberton fell back into Vicksburg, where Grant followed and soon bottled the Confederates. A six-week siege ensued, and Vicksburg fell on July 4.