History is the lie commonly agreed upon - Voltaire Alternative (or alternate) history is a genre of literature that seeks to illustrate and explain a counterfactual: what the world would look like based on circumstances relating to the real world remaining constant except for a few vital differences.

Alternate history does not include stories set in the future (i.e. in an era subsequent to the writer's own period), like Buck Rogers or Neville Shute's On the Beach. Instead, it works in within an existing historical narrative that both the reader and writer are familiar with. In most cases a character in an alternate history novel might describe a critical set point in history where previously the read world and the alternative world were essentially identical, and subsequently everything went pear-shaped. However in other novels the absence of an explanation can actually make the world seem more harrowing.

In my opinion, the best alternative history novels (and the most pure examples of this genre) are those that present the most plausible storylines, in a world that readers can almost relate to. However, several alternate history books involve science fiction to some extent, in particular the novels by Harry Turtledove. In some cases the nature of parallel universes is explored with characters moving between the two (such as the Back to the Future trilogy), while in others (particularly those not involving fantasy), the alternative universe is presented as the only timeline. Alternative histories which are easy to relate to actually can assist readers in understanding the significance of real history, or the societies they live in (consider White Man's Burden). Several are written as mystery novels, where both the reader and the main character slowly fit the pieces of an unverifiable history together.

An Alternate History of the World

528 - Two aliens arrive on Earth. One coaxes an African warlord to dominate the world, while the other seeks to stop them by recruiting the Byzantium and the Holy Roman Empires (An Oblique Approach ).

600 - The Roman Empire still exists, and is poised to enter into a final battle with the Persians (The Shadow of Ararat )

1588 - England is ruled by the Spanish, whose armada sunk the Royal Navy (Ruled Britannia)

1681 - Sir Isaac Newton ditches science and discovers sorcery (Newton's Cannon )

1855 - Charles Babbage's first attempt at a computer does a lot more than simple arithmetric, and thus changes the entire character of the Victorian era ( The Difference Engine )

1884 - Native Americans form their own country Apacheria and bring to a crashing halt the westwards destiny of the American colonialists (Apacheria)

1776 - King George III and George Washington amicably settle their differences, and for the next two hundred years the United States remains a kindler, gentler colony of England (The Two Georges)

1881 - The Confederates win the American Civil War, forcing Abraham Lincoln from office, where he becomes an itinerant socialist rabble rouser (How Few Remain)

1937 - Oswald Mosely comes to power in England (The Leader)

1941 - Britain is ruled by Germany (SSGB, and heaps of other novels)

1942 - Adolf Hitler is temporarily put out of action after a plane crash, allowing for some smarter heads in the Wehrmacht to take control and wage a better outcome on the African and Eastern fronts (The Moscow Option)
Germany and Japan defeat and occupy the United States (The Man in the High Castle)

1946 - The United States, which has crushed Japan, and Nazi Germany, which rules Europe, confront each other with nuclear weapons (1945 co-authored by Newt Gingrich)

1964 - Germany is still fighting the Soviet Union half way to Vladivostok while seeking a peace treaty with President Joseph Kennedy of the United States. Meanwhile a Nazi police officer undercovers secret documents that might help explain why Jews seem to be thin on the ground in Europe (Fatherland).

1972 - Neither Khrushchev nor Kennedy blinked ten years earlier, and in the radioactive, post-Cuban War ruins of the United States life somehow goes on. (Resurrection Day)