World For World Is Forest by Ursula K. Le Guin

When the inhabitants of a peaceful world are conquered by the 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, led by Selver, to retaliate against their captors, abandoning their strictures against violence. But in defending their lives, they have endangered the very foundations of their society. For every blow against the invaders is a blow to the humanity of the Athsheans. And once the killing starts, there is no turning back.