The Word for World is Forest is an explicit allegory for European expansion into new territories and specifically British expansion into North America. From a postcolonialist perspective, the novel treats expansion as generally negative, since it involves slavery, exploitation of resources, and accepted brutalities and atrocities in the name of progress.
A good example can be found in the beginning of the novel:
Ben, his creechie, had the water ready and steaming over the fire, as usual. Creechies never slept, they just sat and stared. "Breakfast. Hurry-up-quick!" Davidson said, picking up his razor from the rough board table where the creechie had laid it out....
(Le Guin, The Word for World is Forest, Google Books)
This draws many parallels to European Imperialism and expansion to both the Americas and Africa; Ben, the alien servant, is treated much like the slaves and servants drawn from indigenous peoples. He is spoken to in pidgin English, he has little autonomous function, and his people are enslaved, ignored, or massacred without moral worry.
The colonizers of the distant planet -- which takes its name from the local word for "forest" instead of "dirt/Earth" because of the alien symbiosis with their ecology -- treat the planet as merely a resource to be used up. Their disregard for the sentient peoples living on the planet is similar to British and later U.S. disregard for Native Americans. Their struggle for dominance is echoed in history; for years, Native American tribes fought against expanding settlers who believed in Manifest Destiny and had no regard for the people already living on those lands. In this sense, the novel allows the natives to eventually win the war, but at a heavy cost; they have learned to kill for benefit from the humans, and might never recover their original, untainted society.