Chickamauga:

Lincoln thought Buell was too cautious and slow, so he replaced him with General William S. Rosecrans. In August 1863, accompanied by an army of about 55,000 troops, Rosecrans advanced to Chickamauga. Bragg, trying to keep his army free for fighting, evacuated the town and went southward, into Georgia as Rosecrans chased him recklessly. Meanwhile, Virginian reinforcements boosted Bragg's numbers to seventy thousand.

During September 19 and 20, Bragg ambushed Rosecrans' army ferociously in the Battle of Chickamauga. The Union right flank crumbled and was soon destroyed. Only the left flank commanded by General George H. Thomas stood firm. General Thomas earned the nickname "the rock of Chickamauga" after the battle. Severely beaten, Rosecrans and his army eventually retreated into the town of Chattanooga.

Chattanooga:

Bragg, although losing 17,000 men at Chickamauga, advanced and overtook Lookout Mountain and Missionary Ridge without many casualties. From these hills, Bragg placed heavy artillery that could easily attack and destroy anything coming from the roads or the Tennessee River to supply Chattanooga. Starvation now threatened the Union army in Chatanooga, but the North had sufficient men west of the town to destroy any threat. Grant, who now commanded all western forces, replaced Rosecrans with Thomas. Then he advanced to Chatanooga with his own army, the Army of the Tennessee.

On November 23-25, the Northerners smashed Bragg's army to pieces during a ferocious encounter in the Battle of Chattanooga. Lookout Mountain fell on the first day. On November 25, an unordered charge up Missionary Ridge was undertaken by Thomas' Army of the Cumberland who were anxious to revenge its defeat at Chickamauga. This successful assault ended the battle in under an hour. The Union now controlled Chattanooga. This was a significant military victory because from here Union armies could quickly move into Georgia and Alabama with ease, thus splitting the Confederacy in two.