Confederate victory in the Civil War is probably the favorite subject of alternate history writers. Even Winston Churchill wrote one, If Lee Had Not Won the Battle of Gettysburg? The title of course shows typical Churchillian irony, and is written from the viewpoint of a historian in the alternate world where Lee did in fact win and the Confederacy gained independence.

Harry Turtledove, the consensus “master of alternate history,” has written two unrelated works based on a Confederate victory; Guns of the South (which involves intervention by time travelers who provide Lee with AK-47s) and the lengthy (11 volume) “Timeline 191” series that starts with How Few Remain and ends with the Union and Confederacy fighting a WW2 analog in the 1940s.
One of the best works in this category, imho, is Ward Moore’s Bring the Jubilee, written in 1953 when there were still living survivors of the Civil War.
All three of these writers portray the long term results as bleak, if not catastrophic. Now, obviously a science fiction writer can create any outcome desired but Churchill and Turtledove in particular are known for their profound historical insight. To a well-informed reader their negative outcomes are quite plausible and almost no qualified historian has written an alternate history in which southern victory is portrayed as a positive good.