There is an old novella from the 1930s called
Lest Darkness Fall that is probably one of the first alternative Roman history science fiction stories. If you're interested in this thread, you would probably be interested in this story. This involves only one individual mysteriously transported back in time. The protagonist of "Lest Darkness Fall" finds himself in Italy during late antiquity after the fall of the Western Roman Empire. Justinian (Emperor of the Eastern Roman Empire) is trying to reconquer and reestablish the Western Roman Empire and the protagonist gets himself involved with the Lombards fighting against the Byzantines. I always thought that this story would make a great movie. However, the time setting would probably have to be changed since few people are familiar with the history after the fall of the Roman Empire. Most of what people know about the Roman Empire is from "Gladiator."