Reap the Whirlwind

Reap the Whirlwind, C.J. Cherryh and Mercedes Lackey, Baen Books, 1989

Part of a barbarian fantasy series that takes place sometime after the fall of the Roman Empire, it's about the adherents of a religion called the Church of Knowledge. Keeping to themselves in a world of cannons and magic, they keep an uneasy peace with their neighbors.

One day, a group of nomads called the Vredai show up in the area. The first thought is that they are marauders planning to wipe this branch of the Church of Knowledge right off the map. Actually, the Vredai are fleeing a group of real marauders called the Talchai who nearly wiped them off the map. 

The two leaders get together, and, realizing that they have a lot to learn from each other, form an alliance and get ready for the coming of the Talchai. But, there are those in both camps who are not happy with the new arrangements, and aim to do something about it. 

It's kind of an old concept, and, as I mentioned, it's part of a series, so there isn't an absolute conclusion at the end. But in this particular case, in the hands of two people who know their way around a fantasy novel like Cherryh and Lackey, it works really well.