The Battle of Chickamauga, fought on September 1920, 1863, between U.S. and Confederate forces in the American Civil War, marked the end of a Union offensive, the Chickamauga Campaign, in southeastern Tennessee and northwestern Georgia. It was the first major battle of the war fought in Georgia, the most significant Union defeat in the Western Theater, and involved the second-highest number of casualties after the Battle of Gettysburg.

The battle was fought between the 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 Union 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 Union army, including Rosecrans himself, from the field.

Union 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.

The American Civil War (April 12, 1861 – May 9, 1865; also known by other names) was a civil war in the United States between the Union (states that remained loyal to the federal union, or "the North") and the Confederacy (states that voted to secede, or "the South"). The central cause of the war was the status of slavery, especially the expansion of slavery into territories acquired as a result of the Louisiana Purchase and the Mexican–American War. On the eve of the Civil War in 1860, four million of the 32 million Americans (~13%) were enslaved black people, almost all in the South.The practice of slavery in the United States was one of the key political issues of the 19th century. Decades of political unrest over slavery led up to the Civil War. Disunion came after Abraham Lincoln won the 1860 United States presidential election on an anti-slavery expansion platform. An initial seven southern slave states declared their secession from the country to form the Confederacy. Confederate forces seized federal forts within territory they claimed. The last-minute Crittenden Compromise tried to avert conflict but failed; both sides prepared for war. Fighting broke out in April 1861 when the Confederate army began the Battle of Fort Sumter in South Carolina, just over a month after the first inauguration of Abraham Lincoln. The Confederacy grew to control at least a majority of territory in eleven states (out of the 34 U.S. states in February 1861), and asserted claims to two more. Both sides raised large volunteer and conscription armies. Four years of intense combat, mostly in the South, ensued.

During 1861–1862 in the war's Western Theater, the Union made significant permanent gains—though in the war's Eastern Theater the conflict was inconclusive. On January 1, 1863, Lincoln issued the Emancipation Proclamation, which made ending slavery a war goal, declaring all persons held as slaves in states in rebellion "forever free." To the west, the Union destroyed the Confederate river navy by the summer of 1862, then much of its western armies, and seized New Orleans. The successful 1863 Union siege of Vicksburg split the Confederacy in two at the Mississippi River. In 1863, Confederate General Robert E. Lee's incursion north ended at the Battle of Gettysburg. Western successes led to General Ulysses S. Grant's command of all Union armies in 1864. Inflicting an ever-tightening naval blockade of Confederate ports, the Union marshaled resources and manpower to attack the Confederacy from all directions. This led to the fall of Atlanta in 1864 to Union General William Tecumseh Sherman and his march to the sea. The last significant battles raged around the ten-month Siege of Petersburg, gateway to the Confederate capital of Richmond.

The Civil War effectively ended on April 9, 1865, when Confederate General Lee surrendered to Union General Grant at the Battle of Appomattox Court House, after Lee had abandoned Petersburg and Richmond. Confederate generals throughout the Confederate army followed suit. The conclusion of the American Civil War lacks a clean end date: land forces continued surrendering until June 23. By the end of the war, much of the South's infrastructure was destroyed, especially its railroads. The Confederacy collapsed, slavery was abolished, and four million enslaved black people were freed. The war-torn nation then entered the Reconstruction era in a partially successful attempt to rebuild the country and grant civil rights to freed slaves.

The Civil War is one of the most studied and written about episodes in the history of the United States. It remains the subject of cultural and historiographical debate. Of particular interest is the persisting myth of the Lost Cause of the Confederacy. The American Civil War was among the earliest to use industrial warfare. Railroads, the telegraph, steamships, the ironclad warship, and mass-produced weapons saw wide use. In total the war left between 620,000 and 750,000 soldiers dead, along with an undetermined number of civilian casualties. President Lincoln was assassinated just five days after Lee's surrender. The Civil War remains the deadliest military conflict in American history. The technology and brutality of the Civil War foreshadowed the coming World Wars.