Complex sentence: The Battle of Chickamauga, fought on September 18--20,1863 between Union and Confederate forces in the American Civil War, marked the end of a Union offensive in southeastern Tennessee and northwestern Georgia--the Chickamauga Campaign.
Split into simpler sentences: The Battle of Chickamauga, fought September 18--20,1863, was fought between Union and Confederate forces in the American Civil War. It marked the end of a Union offensive in southeastern Tennessee and northwestern Georgia called the Chickamauga Campaign.