History sometimes seems to hesitate. It is then that the future of a nation, or even the world, hangs in delicate balance and may be radically altered by a single decision, by the outcome of a battle, or by an unexpected discovery.

What if things had turned out differently? What if Pontius Pilate had pardoned Jesus? What if the Arabs had defeated Charles Martel in the Battle of Poitiers? What if Louis XVI had succeeded in escaping France, and Napoleon III prevented the war of 1870? What if Germany had won the First World War in a month? What if America had not dropped the atom bomb on Japan?

In all these alternatives, we can easily imagine how different long-term history and daily life would have been.

Known as counterfactual or virtual history, this type of historical exercise has become popular in the United States, where such works may become steady long sellers, with sales in the hundreds of thousands. It could be said that counterfactual historiography applies chaos theory (and the fluttering of the proverbial butterfly's wings) to the past. Herodotus and Thucydides, as well as Hegel and Raymond Aron, were fascinated by this approach. Much like a game with rules that must be followed, virtual history allows us to create and invent, and to put imagination in the service of history. As an exercise in speculation, it allows us to understand the mechanisms governing decisive choices, without eschewing the pleasures of narrative and of emotion.

Counterfactual history provides an alternative way of understanding the past: by imagining what could have been we are given a greater insight into what really happened at certain key moments in history. Written by two respected historians known for their accessible style, this book offers readers the pleasure of a lively historical narrative while urging them to enter into the game of reinventing the great events of the past.

Fabrice d'Almeida, a lecturer at the University of Paris-II, was formerly the director of the Institut d'Histoire du Temps Présent (Institute of Contemporary History). His book High Society in the Third Reich first brought him to public attention. He has been a historical consultant on numerous programmes for French television.

Anthony Rowley teaches at the Institut d'Etudes Politiques, in Paris. He is the author of some fifteen books, including Histoire mondiale de la table (2006), and the co-director of a series of historical broadcasts for French television.

Together the authors presented a popular televised series of history programmes, in 2007-2008.