Kim Stanley Robinson uses Alternative History, as a genre, to create a world where European cultures have no influence after 1405 in The Years of Rice and Salt. In order to understand Robinson's methods used in the book it helps a great deal to understand the history of this genre. This genre has been used constantly, in many different works, to explore what could have happened. Only by learning how this genre is written and by understanding the question "What if historical events occurred in a completely different way?" can we really understand why this novel was written.

According to Wikipedia.org (the free encyclopedia):

History of alternate history fiction

Antiquity

The earliest example of alternate history appears to be Book IX, sections 17-19, of Livy's History of Rome from Its Foundation. Livy contemplates the possibility of Alexander the Great expanding his father's empire westward instead of eastward and attacking Rome in the 4th century BC. His main question is: "What would have been the results for Rome if she had been engaged in war with Alexander?"[1]

19th century

In the English language, the first known complete alternate history is Nathaniel Hawthorne's short story "P.'s Correspondence", published in 1845. It recounts the tale of a man who is considered "a madman" due to his perceiving a different 1845, a reality in which long-dead famous people are still alive such as the poets Burns, Byron, Shelley, and Keats, the actor Edmund Kean, the British politician George CanningNapoleon Bonaparte.
The first novel-length alternate history in English would seem to be Castello Holford's Aristopia (1895). While not as nationalistic as Napoléon et la conquête du monde, 1812-1823, Aristopia is another attempt to portray a utopian society. In Aristopia, the earliest settlers in Virginia discover a reef made of solid gold and are able to build a Utopian society in North America.
Early 20th century and the era of the pulps
Although a number of alternate history stories and novels appeared in the late 1800s and early 1900s, the next major work is perhaps the strongest anthology of alternate history ever assembled. In 1931, British historian Sir John Squire collected a series of essays, many of which could be considered stories, in If It Had Happened OtherwiseLouis XVI Had Had an Atom of Firmness." The essays range from serious scholarly efforts through Hendrik Willem van Loon's fanciful and satiric portrayal of an independent 20th century Dutch city state on the island of Manhattan.
Four of the fourteen pieces examined the two most popular themes in alternate history prior to the Second World War: Napoleon's victory and the American Civil War. One of the entries in Squire's volume was Winston Churchill's "If Lee Had Not Won the Battle of Gettysburg"[2], written from the viewpoint of a historian in a world where the Confederacy had won the American Civil War, considering what would have happened if the North had been victorious (in other words, a character from an alternate world imagining a world more like the real one we live in, although not necessarily getting all the details right). This kind of speculative work which posts from the point of view of an alternate history is variously known as a "recursive alternate history", a "double-blind what-if" or an "alternate-alternate history". Other authors appearing in Squire's book included Hilaire Belloc and André Maurois.
Another example of alternate history from this period (and arguably the first to explicitly posit cross-time travel from one universe to another as anything more than a visionary experience) was H.G. Wells' Men Like Gods (1923) in which several British politicians are transferred via an accidental encounter with a cross-time machine into an alternate universe in which Britain had changed course in earlier centuries and developed into a seemingly pacifistic and utopian society. When the politicians from our world try to seize power, the utopians simply point a ray gun at them and send them on to someone else's universe. Wells works out the entire multiverse-pancake framing complete with paratime travel machines that would become popular with U.S. pulp writers (see below), but since his hero experiences only a single alternate world this story is not very different from conventional alternate history (the intruders from our world cause no significant change in the world they enter and are really just a device for examining the results of a past divergence between Wells' utopia and our own world).
The 1930s would see alternate history move into a new arena. The December 1933 issue of Astounding published Nat Schachner's "Ancestral Voices". This was quickly followed by Murray Leinster's "Sidewise in Time". While earlier alternate histories examined reasonably straight-forward divergences, Leinster attempted something completely different. In his "world gone mad", pieces of Earth traded places with their analogs from different timelines. The story follows Robinson College Professor Minott as he wanders through these analogs, each of which features remnants of worlds which followed a different history.

Time travel as a means of creating historical divergences

This period also saw the publication of the time travel novel Lest Darkness Fall by L. Sprague de Camp, which was similar to Mark Twain's A Connecticut Yankee in King Arthur's Court but sent an American academic to the Italy of the Ostrogoths at the time of the Byzantine invasion led by Belisarius. De Camp's work is concerned with the historical changes wrought by his time traveler, Martin Padway, thereby making the work an alternate history. Padway is depicted as making permanent changes and implicitly forming a new time branch (in contrast to Twain's hero, who ultimately fails, with the result that history reverts to its "normal" course).
Time travel as the cause of a point of divergence (creating two histories where before there was one, or simply replacing the future that existed before the time traveling event) has continued to be a popular theme over the decades. In Bring the Jubilee, by Ward Moore, the hero, who lives in a backward world in which the South won the Civil War, travels through time and brings about an alternate history in which the North won at Gettysburg. Ray Bradbury's A Sound of Thunder creates a scenario in which the time travelers inadvertently destroy all history as we know it.
When a story's assumptions about the nature of time necessitate, as in the Bradbury tale, a replacement of the visited historical time's future rather than just the creation of a new time line, the next step is obviously the founding of a time patrol (a device that is not to be confused with the paratime police, see below). Such an agency has the grim task of saving civilization every day, every hour, with patrol members—depicted most notably in Poul Anderson's Time Patrol—racing uptime and downtime to preserve the "correct" history.
This can lead to terrible moral dilemmas. In Delenda Est, the interference of time-travelling outlaws causes Carthage to win the Second Punic War and destroy Rome. As a result, there is a completely different Twentieth Century - "not better or worse, just completely different". The hero, Patrol Agent Manse Everard, must return to that period, fight the outlaws and change history back, restoring his (and our) familiar history - but only at the price of totally destroying the world which has taken its place, and which is equally deserving of existence. The stakes are the highest imaginable: billions of lives balanced against other billions of lives, for one man to decide. "Risking your neck in order to negate a world full of people like yourself" is how the hero describes what he eventually undertakes.
Of course not all time travel stories involve alternate histories. The writer may ignore the possibility of change, or have the cause-and-effect work out so that the time traveler's actions cause the future he remembers, as in Harry Harrison's The Technicolor Time Machine.

(From Wikipedia "Alternative History" article, accessed 02/20/07, GNU Free Documentation License)