The Battle of Chickamauga, fought on September 18 – 20, 1863, between U.S. and Confederate forces in the American Civil War, marks the end of a Union offensive in southeastern Tennessee and northwestern Georgia — the Chickamauga Campaign.

It is the first major battle of the war fought in Georgia, the most significant Union defeat in the Western Theater, and involves the second-highest number of casualties after the Battle of Gettysburg.

The battle is fought between the Army of the Cumberland under Maj. Gen. William Rosecrans and the Confederate Army of Tennessee under Gen. Braxton Bragg, and is named for Chickamauga Creek, which meanders near the battle area in northwest Georgia (and ultimately flows into the Tennessee River about three and a half miles (five point six kilometers) northeast of downtown Chattanooga).

After his successful Tullahoma Campaign, Rosecrans renews the offensive, aiming to force the Confederates out of Chattanooga

In early September, Rosecrans consolidates his forces scattered in Tennessee and Georgia and forces Bragg's army out of Chattanooga, heading south.

The Union troops follow it and brush with it at Davis's Cross Roads.

Bragg is determined to reoccupy Chattanooga and decides to meet a part of Rosecrans's army, defeat it, then move back into the city.

On September 17 he heads north, intending to attack the isolated XXI Corps.

As Bragg marches north on September 18, his cavalry and infantry fight with Union cavalry and mounted infantry, which are armed with Spencer repeating rifles.

Fighting begins in earnest on the morning of September 19. Bragg's men strongly assault but cannot break the Union line.

The next day, Bragg resumes his assault

In late morning, Rosecrans is misinformed that he has a gap in his line.

In moving units to shore up the supposed gap, Rosecrans accidentally creates an actual gap, directly in the path of an eight-brigade assault on a narrow front by Confederate Lt. Gen. James Longstreet, whose corps has been detached from the Army of Northern Virginia

In the resulting rout, Longstreet's attack drives one-third of the Union army, including Rosecrans himself, from the field.

Union units spontaneously rally to create a defensive line on Horseshoe Ridge ("Snodgrass Hill"), forming a new right wing for the line of Maj. Gen. George H. Thomas, who assumes overall command of remaining forces.

Although the Confederates launch costly and determined assaults, Thomas and his men hold until twilight

Union forces then retire to Chattanooga while the Confederates occupy the surrounding heights, besieging the city.