COUNTER-FACTUAL HISTORY is in fashion. The trend owes much of its current momentum to the commercial success of Virtual History, a volume of 'what if?' scenarios published in 1997. With the help of a closely argued introduction by its editor, Niall Ferguson, the book won respectability for a method that had until then been disregarded by many academics as the last redoubt of those who had failed in broadsheet journalism.

Certainly, the case against a counter-factual approach was not difficult to make. The historian's task, it was wearily pointed out, was to uncover what happened and why, not to play Mystic Meg with alternative endings and imaginary consequences. But many of the most dismissive critics of the technique also peddled