An Oblique Approach by David Drake and Eric Flint
The first in the Belisarious series. Alternate History by by two masters of the genre. Forces of vast power in the distant future locked battle, and decide to “fix the fight” by sending agents back in time to change history. The first wants to lock humanity into their vision of perfection. They send an AI back to days of the Eastern Roman Empire to create a powerful empire in Northern India, using their caste system to their advantage. Here is a hint for those who haven’t read a lot of work by Drake, it involves introducing gunpowder long before it was used for weapons systems in our timeline. The second group sends a crystalline intelligence to contact a Roman General by the name of Belisarius. Conflict on a grand scale follows. A series of good adventure stories.