Summer, 1863. The Federal triumph at Vicksburg has secured complete control of the Mississippi River from the Confederacy, cementing the reputation of Ulysses S. Grant. Farther east, the Federal army under the command of William Rosecrans captures the crucial rail hub at Chattanooga. But Rosecrans is careless, and while pursuing the Confederates, the Federal forces are routed in north Georgia at Chickamauga Creek. Retreating in a panic back to Chattanooga, Rosecrans is pursued by the Confederate forces under General Braxton Bragg. Penned up, with their supply lines severed, the Federal army seems doomed to the same kind of defeat that plagued the Confederates at Vicksburg. But a disgusted Abraham Lincoln has seen enough of General Rosecrans. Ulysses Grant is elevated to command of the entire theater of the war, and immediately replaces Rosecrans with General George Thomas. Grant gathers an enormous force, including armies commanded by Joseph Hooker and Grant's friend, William T. Sherman. Grant's mission is clear: Break the Confederate siege and destroy Bragg's army. Meanwhile, Bragg wages war as much with his own subordinates as he does with the Federals, creating dissension and disharmony in the Southern ranks, erasing the Confederate army's superiority at exactly the wrong time.