Disasters threatened Vicksburg on both ends of the Mississippi River early in the war. South of the mighty bastion, New Orleans--one of the Confederacy's most important cities--fell in April 1862. Memphis to the north fell two months later. Vicksburg, Mississippi, stood defiantly between the captured cities atop 300-foot bluffs that dominated the wide mighty river. So long as "the Hill City" remained in Rebel hands, the Confederacy controlled river...