The Word for World is Forest by Ursula K. Le Guin

A world of peaceful aliens conquered by bloodthirsty yumens, their existence is irrevocably altered. Forced into servitude, the Athsheans find themselves at the mercy of their brutal masters.

Desperation causes the Athsheans to retaliate against their captors, abandoning their strictures against violence. In defending their lives, they endanger the very foundations of their society. Every blow against the invaders is a blow to the core of Athsheans’ culture.

And once the killing starts, there is no turning back.

(Orion)