Effectively, this means that what we know as the United States was split into two countries, one in which slavery remained legal.
The area where the two countries meet is called the Mason-Dixon Demilitarised Zone (not unlike the DMZ that separates North and South Korea) and the show will follow characters including politicians, slave hunters, abolitionists, journalists and freedom fighters in the lead up to the "Third American Civil War".
One fan blasted it as "a racist's wet dream".
Benioff and Weiss will serve as showrunners and writers for the series, like they do for Game of Thrones. The show won't go into production until after the final season of Game of Thrones next year.
Benioff and Weiss have brought on board Nichelle Spellman (The Good Wife, Justified) and Malcolm Spellman (Empire) as writers and executive producers.
While Game of Thrones was adapted from George R.R. Martin's books, Confederate appears to be an original concept.
"We have discussed Confederate for years, originally as a concept for a feature film," Benioff and Weiss said in a statement. "But our experience on Thrones has convinced us that no one provides a bigger, better storytelling canvas than HBO. There won't be dragons or White Walkers in this series, but we are creating a world."
However, within hours of the announcement, the backlash was on. Social media users took offence at a fantasy series in which a modern American society still enslaved African-Americans, especially given the current political atmosphere in the US.
The backlash follows criticism from Star Wars actor John Boyega, who recently slammed Game of Thrones for its lack of diversity in its cast.
