The Word for World Is Forest is a science fiction novel by Ursula K. Le Guin, published in 1976...
Several centuries in the future, humans from Earth have established a logging colony and military base named "New Tahiti" on a tree-covered planet whose...big-eyed inhabitants have formed a culture centered on lucid dreaming. Terran greed spirals around native innocence and wisdom, turning the ancient society upside down.
Colonists from Earth take over a planet…They follow the 19th century model of colonization: cutting down trees...building mines…The natives are ill equipped to comprehend this, since they're a subsistent people who rely on the forests, and have no cultural precedent for tyranny, slavery, or war. The invaders take the land of these tiny forest people without any resistance.
Earth has suffered some environmental disasters…The military culture has some familiar aspects, but there have been cultural shifts.’’
[The bad guy] secretly organizes a raid and mass slaughter of a nearby Athshean tree-city. The Athsheans respond by staging a massive raid on "Central", the main Terran base, which they manage to overrun.