The battle of chickamauga was considered a union victory because the union army seized control

Biggest battle ever fought in Georgia, took place on September 18-20, 1863, during the Civil War (1861-65). With 34,000 casualties, it is generally accepted as the second bloodiest engagement of the war; only the Battle of Gettysburg in Pennsylvania, with 51,000 casualties, was deadlier. The campaign that brought the Union and Confederate armies to Chickamauga began in late June 1863, when the Union Army of the Cumberland under Major General William S. Rosecrans advanced southeastward from Murfreesboro, Tennessee, against the Confederate Army of Tennessee, commanded by General Braxton Bragg. Rosecrans's goal was to capture the city of Chattanooga, Tennessee, an important rail junction and gateway to the Deep South. Through a series of successful and relatively bloodless turning movements, Rosecrans's army forced Bragg's troops to abandon middle Tennessee and fall back to Chattanooga.Hope That Helps :)