"War is cruelty, and you cannot refine it. Its glory is all moonshine. Only those who have neither fired a shot nor heard the shrieks and groans of the wounded cry aloud for blood, vengeance, and desolation. War is as close to hell as a merciful God allows upon this earth." -- How Few Remain by Harry Turtledove
Alternate realities have been a staple of science fiction for a long time with the idea that an infinite universe must also consist of limitless mirror versions. Time travel plotlines also provide the fodder for paradoxes in which the slightest (or not so slight) change in history creates alternate timelines.
The alternate reality plot device has been very popular lately. Watchmen, Star Trek, Fringe, and Lost have all dealt with it in one way or another.
The master of the alternate history genre is without a doubt Harry Turtledove. He's a prolific writer who has explored the "what ifs" of history in a wide range of stories. What if the South was victorious in the American Civil War? What if the legendary continent of Atlantis really existed and never sank into the ocean? My favorite is his "Worldwar/Colonization" series in which he examines what might have happened if World War II had been interrupted by an alien invasion.
Turtledove's imagination has led him to some interesting scenarios like retelling the events of World War II in a fantasy world of dragons and magic or, in probably his most popular novel The Guns of the South, writing a tale about time travelers who introduce AK-47s to the 1800s in an effort to alter the outcome of the Civil War. Such stories might seem frivolous, but the genre, when done right, as Turtledove often does, can be fascinating and powerful as it makes the reader see the events of history or the present in a fresh, new light.
So if you're a fan of that type of fictional speculation, you can't go wrong by reading the works of Harry Turtledove.