Author: C. J. Cherryh.
Title: The Faded Sun Trilogy.
Copyright: 1978, 1979 (2000, paperback).
The mercenary mri lose a war with humanity. The mri's former employer, the regul, betrays them on the desert world of Kesrith. The only survivors are a brother and sister who are joined by a human soldier to retrace the mri's footsteps back to their home-world.
The mri themselves are an honor bound caste of priests, warriors, and civilians. The novel take off once the three leave Kesirth for the mri home-world.
On a dying desert world, they encounter the other mri who never left for the stars. The survivors have regressed and no longer live with technology. It takes the outsiders to show them the benefits of the planet wide AI that still exist (along with planetary defenses).
But the regul show up to finish off the mri. However, humans intervene. The regul fleet is destroyed in orbit and the mri reach a compromise with humanity. They promise to serve as mercenaries in exchange for leaving the mri home-world alone.
I enjoy C. J. Cherryh's writing. But the desert settings are much like Dune. It took me a long time to finish this book. Not that I didn't try, but life got in the way. I'd hoped to have it read in a month when I started in January. But I've had to deal with a parent who has dementia issues along with other health problems. I can't even begin to say how much time I've spent in ER waiting rooms these past three months.
Alas, I didn't have The Faded Sun Trilogy to read when I had downtime. But I finished it this weekend. Recommended.