Historical “what-if” scenarios abound in our current political, academic, legal, cultural, and literary discourses. This book reveals the origins and development of the counterfactual mode of historical thought, exploring its reasoning, uses, and the analytical and narrative forms it takes. The book traces the actual history of counterfactual history from its eighteenth-century roots in religious debates, moral philosophy, and military sciences to its current applications in fields like economic history and reparative justice. And it explores the historical-counterfactual mode in narrative fictions, from nineteenth-century French utopias and dystopias to contemporary science fiction and alternate-history novels. The majority of the book concentrates on writings that cluster around two long-established counterfactual-historical loci: 1) the American Civil War and its aftermath; and 2) the summer of 1940 and the Battle of Britain, when Great Britain was Nazi Germany’s sole undefeated opponent. The two case studies demonstrate how Americans and Britons have used historical counterfactual speculations and narratives to gain perspective on their current predicaments as well as to structure and revise their national characters.