The Western part of the war had become divided into two main parts. The Union armies won their first object when they gained control of the Mississippi. But another long and bitter contest was fought out before they could secure the strongholds of central Tennessee and northern Georgia.

The first great battle on this line was that of Stone's River, or Murfreesboro, fought on the last day of the year 1862, about the time that General Ulysses S. Grant was beginning operations against Vicksburg. The conflict was marked by the brilliant charges made by the Confederates under General Braxton Bragg, which at length broke to pieces the whole right wing of the Union army. General William Rosecrans had succeeded Don Carlos Buell in command of the Union troops. The result of the day's fighting was very favorable to the Confederates. But in the latter part of the day the half-defeated Union soldiers, under the immediate command of General George Henry Thomas, made the most determined resistance to the dashes which the Confederates continued to make. Some of the generals wished to retreat, but Rosecrans, who had defended Corinth with so much stubbornness, announced his intention to "fight or die here." On the next day, which was the first day of 1863, neither of the shattered armies was in a condition to make a serious attack. On the third day of the battle the Confederates, by a tremendous charge, drove back part of the left wing of Rosecrans's army, but they were soon cut to pieces and themselves driven back. After the two armies had bravely held their ground with varying fortunes for three days, Bragg retreated, and Rosecrans entered Murfreesboro. Each army had lost about nine thousand in killed and wounded, besides those captured.

In the summer and autumn of 1863, Rosecrans, by some well-planned maneuvers, put Bragg at such disadvantage that he was forced to fall back from time to time until he had left Chattanooga in the hands of the Union troops. But Bragg received reinforcements, and the great Battle of Chickamauga was fought on the 19th and 20th of September, 1863. It was a battle of charge and countercharge. On the first day the Union army won considerable advantage; but on the second day the right half of Rosecrans's army was broken, and it retreated in confusion toward Chattanooga. The utter rout of the Union army was prevented by General Thomas, whose division had also saved the army at Murfreesboro. With extraordinary coolness he held the left wing against repeated assaults, and, when ammunition failed, his soldiers used their bayonets to repel the Confederate charges. Though Bragg's troops, by splendid fighting, had gained a great victory, Thomas, by the most brilliant defense of the war, kept them back long enough to enable Rosecrans to prepare for the defense of Chattanooga, to which place the Union troops retreated.

Grant, who had gained great reputation by his Vicksburg campaign, was now given command of all the forces west of the mountains. Rosecrans was relieved, and Thomas, who was called "the Rock of Chickamauga," was put in his place. Grant took immediate command of the besieged troops in Chattanooga, with Thomas next.

Bragg having sent away a part of his army to attack Burnside in East Tennessee, Grant took advantage of this weakening of his force to attack Bragg's army in his front. The main body of Bragg's army was intrenched in Chattanooga Valley. Bragg also held Missionary Ridge, in his rear, and Lookout Mountain, to the southwest. Hooker attacked and carried Lookout Mountain on the morning of November 25, 1863, while a mist shut out the summit from the valley. This is sometimes called "The Battle above the Clouds." But Union General William T. Sherman, who had previously carried an outlying hill at the north end of Missionary Ridge, was checked in his attempt to advance by the obstinate resistance of the Confederates under Hardee. Grant, therefore, sent the army under Thomas out of Chattanooga to attack the Confederates in front, with instructions to carry the first line and lie down. By a swift charge, under a severe fire, they carried the line at the foot of the mountain; but the guns of the Confederates on the top of Missionary Ridge sent a galling fire upon them. Without orders one impatient regiment after another rushed up the hill. Bragg's troops made a vigorous resistance, but the eager assailants carried the line in six places, and the Confederate army was forced to retreat.

Grant was now put in command of all the Union armies, and he took charge in person of the army in front of Washington, while Sherman was left to command the Western army. Sherman, a person of incessant activity and ability of many kinds, was confronted by the Confederate general Joseph E. Johnston, who had been appointed to succeed Bragg. Johnston also was a person of military genius, and extremely prudent. Sherman, by skillful maneuvers, tried to force Johnston to fight in the open field; but Johnston preferred to draw Sherman farther south, so as to increase the difficulty of supplying his army, and to compel Sherman to attack him behind breastworks. Many severe engagements were fought, but Johnston avoided a general battle. At length Sherman attacked Johnston at Kenesaw Mountain, but the Confederates repulsed him.

The Confederate government, dissatisfied with Johnston's long and cautious retreat before Sherman, removed him, and General John Bell Hood took his place. Hood believed in sharp fighting, and several battles took place at various points about Atlanta, but they generally resulted in favor of the Union army. At length, Sherman got a considerable part of his army south of Atlanta, so that Hood was compelled to abandon that city or be shut up in it.