Alternate history is one of those subgenres that's typically associated with science fiction, but there's nothing inherently "science fiction-y" about it. Certainly, you can't claim that it's a part of "normal" non-speculative fiction since its version of history is alternate. In addition, many of its most famous examples, including such classics as Harry Turtledove's The Guns of the South, use science fiction-y elements like time travel to create the alternate realities in which they occur. On the other hand, there are plenty of examples of works in which the only speculative device the author uses is the alternate history itself; one of the most notable of these is the work that I'll be focusing on today: Philip K. Dick's Hugo-winning novel The Man in the High Castle.
In the world of The Man in the High Castle, the Japanese attack on Pearl Harbor completely decimated the US navy, allowing the Japanese to stage an enormously successful invasion of the US mainland, which then allowed the Axis to win World War II. The Japanese and the Reich then engaged in a Cold War against each other, á la our own Cold War, and this sets the stage for the remainder of the book. The novel follows the lives of a few select individuals as they go about their lives in a post-war United States, which has been split between the Japanese, on the West, and the Nazis, on the East. There are a few minor technological differences between their world and ours, mainly due to the differences between American and Nazi technology, but these differences are meaningless except to more fully flesh-out the story's world.
Compare this setting to that of the recent (and excellent) film Inglourious Basterds. It too portrays a version of WWII that did not occur in history. Without spoiling anything for those of you who haven't yet seen it, suffice it to say that it depicts an allied attack on the Nazi high command that has tremendous consequences for the rest of the war. And yet, while the device it uses is pretty much identical to the one used in The Man in the High Castle, no one would ever confuse Inglourious Basterds for science fiction. Chaos would have ensued if it had so much as been nominated for a Hugo, yet that's the same award that The Man in the High Castle won. Why?
As I've said so many times, what matters in science fiction is not the setting itself, but rather how that setting is used. Inglourious Basterds uses its alternate history setting to tell an adventure story, a damn good adventure story, but a simple adventure story nonetheless. The Man in the High Castle, on the other hand...
The main characters in The Man in the High Castle are an antique salesman who believes his antiques might be counterfeit; a German spy posing as a Swedish businessman; and, perhaps most importantly, although he's not a main character, is Hawthorne Abendsen, author of the novel-within-the-novel The Grasshopper Lies Heavy. The Grasshopper Lies Heavy is an alternate history novel in which Germany and Japan lose WWII (if you can imagine). All of these elements (fake objects, fake people, fake realities) come together to create a novel in which questioning what is "real" is integral to the experience of the story. At the very end of the novel, two of the characters use the I Ching to question why it was that The Grasshopper Lies Heavy was written. The I Ching replies with the Chinese symbol for "inner truth." These two characters then conclude that the reality in which they are living–a reality in which Japan and Germany won WWII–is a false one, and that The Grasshopper Lies Heavy contains the actual truth. They are correct, of course; their reality is a fictional one, created by Philip K. Dick for a Hugo-winning novel; in the "real" reality, Japan and Germany really did lose.
But is their reality really the fake one? It's impossible to read The Man in the High Castle without seeing it as the The Grasshopper Lies Heavy for our own reality; implicitly, then, we cannot help but question the world in which we live. Who's to say it's "genuine?" Who's to say that our world is not simply the setting for some other Hugo-winning alternate history novel, a novel that is being written as we speak in a universe that is truer than our own? The Man in the High Castle does not purport to answer these questions, of course. It merely provokes us to ask them.