Murray Leinster's iconic short story ‘Sidewise in Time’, first published in the American magazine Astounding in 1934, is often cited as the point at which the alternate history narrative first enters science fiction as a plot device. It is not a conventional alternate history as we now recognize them, but it is nonetheless significant for demonstrating to fans and writers of speculative fiction (sf), particularly American science fiction (at that time undergoing its crucial metamorphosis in the Golden Age of the sf magazines), that speculation need not restrain itself to ‘what if’ stories predicated on questions of science and technology, but that it may also encompass historical questions. In recognition of this important position within the field, the awards presented annually for best alternate history fiction are named the Sidewise Awards.

As the above quotation from ‘Sidewise in Time’ suggests, what alternate history really represents is nothing less than an unravelling of our linear and singular notions of history, an infinite fracturing of time into infinite pasts, presents, and futures. Alternate history then is not just about history; it is very much also about the present and the future. Indeed, as we shall see throughout the following chapters, the alternate history texts which are the most compelling works of fiction are those which use the alternate world of their creation to alter the reader's perceptions of their own history, present, and potential futures. Beyond mere allegory, the best alternate histories offer an avenue of critique and provocation which exceeds the capacities of their nearest genre neighbours (historical fiction, science fiction, or fantasy).

This is one of the reasons why it would be a mistake to categorize alternate history as purely a sub-genre of science fiction. Whilst much of the familiar iconography of science fiction can find its roots in the pre-genre literature, alternate history narratives have not only a separate genesis but an independent literary history.