I’m re-reading the Belisarius series of six books now. Belisarius was a general in the Byzantine Empire in the 500s, who is most famous for retaking Africa from the Vandals and Italy from the Goths (mostly) during the reign of Justinian with absurdly small forces. The emperor never trusted him, and mistreated him but he never turned on him. He has been a favorite of alternate history buffs since even before there were alt-history buffs.
In this alternate history, two contending forces from the far far future have sent agents back to change the past. The bad guys have sent Link to Malwa, an Indian state, to impose caste restrictions and racial purity ideas on the past. Rome, with its ideas of rule based on ability, is to be destroyed. The good guys send a crystal intelligence, Aide, to Rome to help defeat them.
Aide can give visions of the future that will come if Malwa wins to help convince doubters. When Belisarius sees his future, in the last moments before his death, he sees that an Indian refugee that he had serving him was a hero that had fled when the last princess of the dynasty he served had been killed by the Malwa. If he can change that, and save her, then the Malwa will at least have opposition on the home front.
Belisarius has quite a few problems to solve before this – for one, he needs to defeat the Persians in the war he is in, which hasn’t been done in a century or so. He needs to start research on gunpowder weapons to counter those the Malwa have. And he needs to induce a paranoid Emperor to send him to India without telling him the secret!
Only then can the easy part of rescuing the princess from the middle of a 100,000 man army even begin. It shouldn’t be too hard – there are three Roman bodyguards, five Axumite allies (from Ethiopia), and Belisarius. And he has a plan….