Roberts, Ralph. How the South Won the War and Preserved the Union
Divergence: 1849 CE
What if: In 1849, Zachary Taylor and Millard Fillmore were killed in an accident, elevating the Senate president pro tem to the US presidency.
Summary: David Atchison's presence in the White House provokes the abolitionist North into secession, leading to a different Civil War.
Published: Alexander/Farther Star 1997 (1570900523).
Published: Expanded from "How the South Preserved the Union", in Alternate Presidents (ed. Mike Resnick), q.v.