The central theme behind this story is one of those cocktail-party what-if discussions. What if Napoleon hadn't invaded Russia? What if Columbus had been turned down by Spain? Or, add a technological edge: what if Napoleon had nuclear weapons at Waterloo? What if the South had AK-47's in the Civil War?
Hold it a minute on that last one. This is exactly what Harry considers. Being a science fiction writer and Civil War buff, it's a natural. But Harry considers practical aspects of the problem. For example, you can't just show up with a few AK-47s, you need thousands to make a difference; these aren't nukes after all. And what about training to use them? And what about ammunition; you can't just stuff minie balls into them! Harry addresses all of these issues, successfully, in my opinion.
As if that weren't enough, Harry comes up with a plausible reason WHY someone would want to supply the South with semiautomatic weapons to win the war.
I found that he got the characterizations of both the major and minor characters right. Historical figures like Robert E. Lee, Lincoln, and Nathan Bedford Forest ring true to me. And his trenches-level soldiers are very well-done, too, and drawn from historical figures. And the book is, overall, very well-written. It keeps you interested in what's going to happen next, since it departs from "real" history soon after 1864. It helps to be a Civil War buff, as there are many scenes that give you a flavor of what it was like (which I also thoroughly enjoyed). I can see how that could be a little tedious if one does not have an interest in "The Late Unpleasantness" as Southerners still call it. But unless one really hates this period of history, I think it is well worth reading.