Sauron Dominion (War World Vol. III) by Jerry Pournelle
Born of rebellion and civil war, cut off from the rest of humanity after the Seccession Wars, Haven has been bombed back to a pre-tech medievalism by a race of hideous "supermen" intent on keeping the planet cut off from the rest of the empire while they slowly absorb all normal humanity into their own perverted form. Haven is a world forever at war, each with all - and all against the Saurons.
