Final answer:
A novel that begins in a past time period and then focuses on events in a later time period is using a flashback as a literary device.
Step-by-step explanation:
If a novel opens with a scene in the Civil War, but the book is about a character in the early 1900s, then flashback is the literary device used. A flashback is a plot device where the narrative goes back in time to provide context, background information, or depth to the story's current circumstances. For example, in "An Occurrence at Owl Creek Bridge" by Ambrose Bierce, the story begins in the present with a character facing a dire situation, but it moves back in time to recount the events that led up to this moment. This technique allows writers to introduce past events that are crucial to understanding the current narrative and to develop their characters more fully.