| Pastwatch, an alternate history novel by Orson Scott Card, explores the past from the point of view of a slowly dying post-apocalyptic future. This future is slowly consuming the last of the Earth's resources, looking forward only to the twilight of the human race. A technology breakthrough enables the denizens of the future to view the past, at a very macroscopic layer - weather patterns and such.
An organization (Pastwatch) is formed, to study the past, and learn from it. As technology improves, Pastwatch gains finer grained visibility into the past, able to view at first cultural migrations, then individuals. A chance discovery shows Pastwatch that they can affect people in the past. Since they assume the classic model of time travel, they realize they can change the past ... once. What single thing would you change in history?
The answer is, of course, the discovery of the New World. Exploring the common belief that the single greatest sin in human history was the introduction of slavery to the New World, they set out to determine when to influence Christopher Columbus away from his quest. An in-depth study determines that an alternate, former timeline had already mucked with history - acting to send Columbus to the New World. What crime in history could be worse than the introduction of slavery to the New World?
The introduction of human sacrifice to the Old World. And now, the future plans a utopian, Caribbean society to replace both options. | 
