Here’s some background:

After taking charge of the Union’s western armies in October of 1863, Maj. Gen. Ulysses S. Grant focused on lifting the Confederate siege of Chattanooga, Tennessee, which had been in place since the Battle of Chickamauga in September.  Grant opened the “Cracker Line” across the Tennessee River to bring supplies to the beleaguered Army of the Cumberland inside the city, and, in mid-November, brought Maj. Gen. William T. Sherman’s Army of the Tennessee into the city as well.  The Confederates under Maj. Gen. Braxton Bragg had established themselves on Missionary Ridge and Lookout Mountain, both of which had commanding views of the city. On November 23rd, the reinforced Federals began to fight their way out, overrunning Orchard Knob at the base of Missionary Ridge and gaining a foothold for continued attacks against the Confederate line.  The next day, Grant launched an attack on Lookout Mountain and captured it after six hours of fighting. On November 25th, Grant ordered Sherman to attack Tunnel Hill on the ridge east of the city. While Sherman’s initial attack was a failure, a second attack by Maj. Gen. George H. Thomas managed to completely break the center of the Confederate line along Missionary Ridge. This third victory in three days compelled a Confederate withdrawal and opened up the deep South to a Union invasion. Source: https://www.civilwar.org/learn/civil-war/battles/chattanooga

5 quick facts

1. This spectacular defeat followed in the wake of the last major strategic victory won by Confederate troops. Only two months earlier the Confederates inflicted a crushing defeat on their opponents in the area. The Confederate high command had finally had some sense, and it had sent troops from Virginia to Tennessee to reinforce the area. The move paid off. For once the Confederates outnumbered the Federals. Aided by sheer good luck – the Union left a gap in their line and the Confederate attack happened to pour right through it- they overwhelmed their enemies at the battle of Chickamauga. Unfortunately for the Confederacy their commanding general in the area was incompetent and failed to exploit the victory. Instead of taking the important railroad hub of Chattanooga, he merely laid siege to it.

2. Confederate forces in the area failed to properly cooperate and a large part of the qbesieging force was sent off on a separate and senseless campaign to try and capture Knoxville. This was probably done to give general James Longstreet a chance to have an independent command. It did not go well. The Confederates would never recapture Chattanooga nor Knoxville. Ironic fact: West Tennessee was pro Confederate but controlled by the Union as of 1862, East Tennessee was pro Union (there was less slavery in the area than in the west of the state), but remained under Confederate control until late 1863.

3. The commanding general, Braxton Bragg, was greatly disliked by most of his officers and men. The Confederate president Jefferson Davis liked him so much, and hated potential replacements, like Joseph Johnston and PGT Beauregard so much, that he kept him in charge much longer than a president like Lincoln would have.

Braxton Bragg

4. The Confederates managed to safely retreat into Georgia thanks to the brilliant rear guard action of General Patrick Cleburne. An Irish born officer from Arkansas. He was one of the first to advocate the use of blacks as Confederate soldiers, a proposal that he drew a lot of criticism for, and a plan that was rejected until the very last (March 1865).

Patrick Cleburne

5. The loss of Tennessee paved the way for the Union’s campaign to capture an even more important city, the city of Atlanta. Its capture right before the presidential elections of 1864 secured Lincoln’s reelection. Until Atlanta’s fall Lincoln was convinced he would not be reelected, which could have meant independence for the Confederacy.

Some figures for the battle that cost the Confederacy the populous state of Tennessee: