Contrary to my usual practice, I read the entire Belisarius Series back to back. Blame Emma: she keeps getting Eric Flint books out of the library and slinging them my way.
Belisarius was a real-life sixth-century Roman general based in Constantinople, the capital of the Eastern Roman (Byzantine) Empire.
In the far, far future, two races descended from mankind fight a proxy war to change the past. The ‘New Gods’, repulsed by the mongrel offshoots of the human race, bet on the Malwa empire of India, with its rigid caste system. At ruinous expense, they send a cyborg named Link back to the sixth century. With gunpowder technology, the Malwas quickly conquer the rest of India and start eyeing the Persian and Roman empires.
A crystalline race send one of their number, Aide, back in time to help Rome’s greatest general, Belisarius, fight the Malwa, and keep Earth’s history, as much as possible, on its previous trajectory. Aide’s vast knowledge of our history and technology augment Belisarius’s native cunning and immense grasp of strategy and tactics, and eventually overcome Link and the Malwa.
The books juggle multiple viewpoint characters, weaving together a complex tapestry of plots, spread over six books. The action flows from Constantinople to Persia, Alexandria, the Axumite (Ethiopian) empire, Arabia, and most of all, to India. The characters are for the most part likeable and larger than life. Most, but by no means all, of the Malwa and their allies are repellent. The battles are many and the carnage vast, as the authors get to cherrypick the tactics of Earth’s greatest generals.
In the end, Link’s fatal flaw is its lack of understanding of humanity, that only the soul matters, of the importance of love and redemption.