Clan and Crown — Jerry Pournelle

For the first few years, Rick Galloway and his band of mercenaries were doing well just to survive. They'd been swept off a hilltop in Africa by a flying saucer, and deposited on an alien world where the other inhabitants were human--but from various and unfriendly periods of history, all collected by flying saucer raids.

Rick has faced facts: This planet is going to be home, permanently. And to create a society safe for themselves and the families they are gradually building, they need to do more than just survive. They must convince the others that a unified, peaceful society is better than a collection of warring tribes. Force would not be Rick's chosen method of persuasion--but on a planet where the other dominant culture is one brought straight from ancient Rome, force may be the only way.