The Battle of Chickamauga, fought on September 18–20, 1863, between the United States Army and Confederate forces in the American Civil War, marked the end of a U.S. Army offensive, the Chickamauga Campaign, in southeastern Tennessee and northwestern Georgia. It was the first major battle of the war fought in Georgia and the most significant Union defeat in the Western Theater, and it involved the second-highest number of casualties after the Battle of Gettysburg.

The battle was fought between the U.S. Army's Army of the Cumberland under Maj. Gen. William Rosecrans and the Confederate Army of Tennessee under Gen. Braxton Bragg, and was named for Chickamauga Creek. The West Chickamauga Creek meanders near and forms the southeast boundary of the battle area and the park in northwest Georgia. (The South Chickamauga ultimately flows into the Tennessee River about 3.5 miles (5.6 km) northeast of downtown Chattanooga).

After his successful Tullahoma Campaign, Rosecrans renewed the offensive, aiming to force the Confederates out of Chattanooga. In early September, Rosecrans consolidated his forces scattered in Tennessee and Georgia and forced Bragg's army out of Chattanooga, heading south. The Union troops followed it and brushed with it at Davis's Cross Roads. Bragg was determined to reoccupy Chattanooga and decided to meet a part of Rosecrans's army, defeat it, and then move back into the city. On September 17 he headed north, intending to attack the isolated XXI Corps. As Bragg marched north on September 18, his cavalry and infantry fought with Union cavalry and mounted infantry, which were armed with Spencer repeating rifles. The two armies fought at Alexander's Bridge and Reed's Bridge, as the Confederates tried to cross the West Chickamauga Creek.

Fighting began in earnest on the morning of September 19. Bragg's men strongly assaulted but could not break the US line. The next day, Bragg resumed his assault. In late morning, Rosecrans was misinformed that he had a gap in his line. In moving units to shore up the supposed gap, Rosecrans accidentally created an actual gap directly in the path of an eight-brigade assault on a narrow front by Confederate Lt. Gen. James Longstreet, whose corps had been detached from the Army of Northern Virginia. In the resulting rout, Longstreet's attack drove one-third of the Army of the Cumberland, including Rosecrans himself, from the field.

U.S. Army units spontaneously rallied 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 assumed overall command of remaining forces. Although the Confederates launched costly and determined assaults, Thomas and his men held until twilight. Union forces then retired to Chattanooga while the Confederates occupied the surrounding heights, besieging the city.

In his successful Tullahoma Campaign in the summer of 1863, Rosecrans moved southeast from Murfreesboro, Tennessee, outmaneuvering Bragg and forcing him to abandon Middle Tennessee and withdraw to the city of Chattanooga, suffering only 569 Union casualties along the way. General-in-chief Maj. Gen. Henry W. Halleck and President Abraham Lincoln were insistent that Rosecrans move quickly to take Chattanooga. Seizing the city would open the door for the Union to advance toward Atlanta and the heartland of the South. Chattanooga was a vital rail hub (with lines going north toward Nashville and Knoxville and south toward Atlanta), and an important manufacturing center for the production of iron and coke, located on the navigable Tennessee River. Situated between Lookout Mountain, Missionary Ridge, Raccoon Mountain, and Stringer's Ridge, Chattanooga occupied an important, defensible position.

Although Braxton Bragg's Army of Tennessee had about 52,000 men at the end of July, the Confederate government merged the Department of East Tennessee, under Maj. Gen. Simon B. Buckner, into Bragg's Department of Tennessee, which added 17,800 men to Bragg's army, a total of 69,800 men, but also extended his command responsibilities northward to the Knoxville area. This brought a third subordinate into Bragg's command who had little or no respect for him. Lt. Gen. Leonidas Polk and Maj. Gen. William J. Hardee had already made their animosity well known. Buckner's attitude was colored by Bragg's unsuccessful invasion of Buckner's native Kentucky in 1862, as well as by the loss of his command through the merger. A positive aspect for Bragg was Hardee's request to be transferred to Mississippi in July, but he was replaced by Lt. Gen. D.H. Hill, a general who did not get along with Robert E. Lee in Virginia.

The Confederate War Department asked Bragg in early August whether he could assume the offensive against Rosecrans if he were given reinforcements from Mississippi. He demurred, concerned about the daunting geographical obstacles and logistical challenges, preferring to wait for Rosecrans to solve those same problems and attack him. He was also concerned about a sizable Union force under Maj. Gen. Ambrose E. Burnside that was threatening Knoxville. Bragg withdrew his forces from advanced positions around Bridgeport, which left Rosecrans free to maneuver on the northern side of the Tennessee River. He concentrated his two infantry corps around Chattanooga and relied upon cavalry to cover his flanks, extending from northern Alabama to near Knoxville.