The Word for World is Forest

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.
Winner of the 1973 Hugo award for Best Novella, and nominated for many others, The Word for World is Forest is part of Le Guin's 'Hainish Cycle'. It explores a future history of Earth and pacifistic ideals in its depictions of violence, colonialism and resistance.