Wikipedia

The Battle of Chickamauga, fought September 19–20, 1863, marked the end of a Union offensive in southeastern Tennessee and northwestern Georgia called the Chickamauga Campaign. The battle was the most significant Union defeat in the Western Theater of the American Civil War and involved the second highest number of casualties in the war following the Battle of Gettysburg.

The battle was fought between the Union Army of the Cumberland under Maj. Gen. William Rosecrans and the Confederate Army of Tennessee under Gen. Braxton Bragg, and was named for West Chickamauga Creek, which meanders near the battle area in northwest Georgia (and 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.

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. 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, 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.