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.