The Battle of Chickamauga, fought September 18–20, 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. It was the first major battle of the war that was fought in Georgia.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, 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.