“Chickamauga” is set during the Battle of Chickamauga, a Civil War battle that took place in northwestern Georgia in September of 1863. The American Civil War was fought between the Union (northern states), which wanted to abolish slavery, and the Confederacy (southern states), which wanted to continue the practice and therefore sought to secede from the Union.
While the Union army eventually defeated the Confederates in 1865, the Battle of Chickamauga was one of their more devastating defeats. In fact, it was a Civil War battle with one of the highest number of casualties. It’s likely that Bierce set the story during this specific battle because he experienced it first-hand as a Union army solider and wanted his readers to understand the horrors of his experience.
It is notable that Bierce made the protagonist of the story a southern child whose plantation-owning family enslaved people and, therefore, likely supported the Confederacy. The child interacts with the wounded and dying soldiers the way he is used to interacting with “his father’s negroes” who would “creep upon their hands and knees for his amusement.” It’s likely that Bierce was intentionally criticizing this sort of entitlement that southern white people passed on to their children. Of course, by the end of the story, the child learns an important lesson as he watches his house burn down with his family inside—war brings harm to people regardless of their power and status.