Gallagher locates the origins of contemporary counterfactual history in eighteenth-century Europe, where the idea of other possible historical worlds first took hold in philosophical disputes about Providence before being repurposed by military theorists as a tool for improving the art of war. In the next century, counterfactualism became a legal device for deciding liability, and lengthy alternate-history fictions appeared, illustrating struggles for historical justice. These early motivations-for philosophical understanding, military improvement, and historical justice-are still evident today in our fondness for counterfactual tales. Alternate histories of the Civil War and WWII abound, but here, Gallagher shows how the counterfactual habit of replaying the recent past often shapes our understanding of the actual events themselves. The counterfactual mode lets us continue to envision our future by reconsidering the range of previous alternatives. Throughout this engaging and eye-opening book, Gallagher encourages readers to ask important questions about our obsession with counterfactual history and the roots of our tendency to ask "What if...?"