This one is a trip, let me tell ya. Eric Flint and David Drake seemed to have decided that they want to fix history and this is the result. With a bit of sci-fi twist as a catalyst.
These books focuses on the noble General Belisarius of the Byzantine Empire, who was a real person and considered by many to have been the Last True Roman. If you want to know more about the historical figure, I recommend this book. There aren’t many books about him, but that one is a nice overview.
Anyway, the alternate history of the Belisarius saga begins when in the far, far distant future, the descendants of humanity are locked in a war between a hopeful and just set of…crystal whale things…and an evil, oppressive, and utopian regime. It’s actually rather vague but post-humans almost always are, so if you’ve read some serious space opera, you know how it goes.
The bad guys have figured out that, to win without firing a shot, they need to change history at a certain point and conquer the world and shift the culture to one that is influenced by a particular subculture in India. They send back one of theirs to do so. The good guys learn of this plan and send a sentient crystal to Belisarius to prevent this future, altering history forever.
I can’t really explain this series. It’s just… the crystal tells Belisarius the whole of human military history, essentially, and then you get quite feasible chariot-drawn rocket launchers in the late Byzantine Empire. It’s military SF and entertainingly self-indulgent, somehow. If you are a classicist, this book endeavors to fix history and human development. There’s one thing that happens and the book takes the time to point out that they just prevented the fall of Byzantium and many other awful things (from a classicist’s point of view).
Flint and Drake have a writing style that lends itself to writing strategic battles and operatic politics with big personalities, which is this book. I don’t even know how it got published by a big publishing house, though God bless Baen and the long leash they give their people. Books written by people about using drugs about drugs are less trippy because they are not so grounded as this. There was serious research that went into it; you don’t just recount the culture of Axum (Ethopia) based on a Wikipedia page. It’s crazy and, if you are a military history buff, it’s really entertaining.