Alternate histories examine our world as if it had had experienced a reasonable change in the past. They offer insights into our present existence. When well done, they provide us with a detailed development of the historical possibilites of people in worlds which might have been.
The fact that we can produce plausible alternate histories demonstrates that history is produced by choices rather than inevitable forces that push us towards a predetermined outcome as argued by theorists ranging from St. Augustine, to Marx, Spengler and Fukuyama. The sum of many individual choices can be measured as "trends", "cycles" "progress" and so forth. But these are artificial constructs of to describe what actually happened. They are no more than the sum of the actions of every individual human being, each acting for their interests within the constraints on their scope of action.
Conventional historians drifted out of this field in the early twentieth century, and left most of it to science fiction writers. Previously, historians and essayists exploited the insights that counterfactuals provide to historical analysis. For example, G.M. Trevelyan speculated in 1907 on the historical consequences of a Napoleonic win at the battle of Waterloo.
Alternative history is based on the idea that we can identify a "point of departure" or branch point between our history and an alternative. These are often dramatic decisive events such as battles, wars, and elections that generate clearly different alternatives.
We change the past to learn about the consequences, for better or worse, of actions that might have been taken in the past, and to appreciate the role of chance and luck. But we make our changes in the framework of our understanding of the multitude of small changes that are called forces, trends, or developments, as well as the social and technical constraints of an era.
