Hi, I am a writer in the seedbed of the southern confederacy of states (aka South Carolina). I once read a short paperback novel with a title "If the South had won" or something to that effect. Though I have never tried to write a short story, much less a novel, with that theme, I am not surprised to see that is a favorite alternate history subject.
I have always liked historical fiction, but it comes with the limiter that what your characters do must mesh with what really happened. If you introduce in fiction something that would inadvertently alter the timeline, then that section becomes a liability. This is a problem also in building a back story for science fiction. A case in point is the future of the original "Star Trek." Written in the early sixties, it projected a scenario of things they suspected might happen in the nineties. When the franchise remained popular into that time period, histories had to be changed. Of course, with the preponderance of time travel in that franchise, it has proved to be less of an problem.
Alternate histories, though, need only to create a situation that can cascade out of control if changed. The alternate universe theory of quantum physics has made this a popular device (e.g. "Sliders"). In the past, several time travelers have found themselves in place to CAUSE history to go the "right" way to prevent alternate timelines from happening. Sometimes, though, the alternate history is "wiped out" when a change in its past is introduced (not a logical consequence, to be sure).
The "alternate history" genre gives the historical novelist the freedom to "make things up" based on his own research. The consequences of changes can be controlled, to an extent, by adding enough variables. The book I read on the southern victory basically kept the states as they were, with two countries on the same continent. The war ended early, allowing certain characters who died in the war to live and have children who became leaders in the subsequent history. After a century of good relations, they reunited in 1963.
Though this does not relieve the writer from doing his research, it gives him the creativity to "make things happen" in a universe largely "in his control." Critics can still pick it apart, but if the research is sound, the changes can be sustained.