Human colonists land on a remote planet, intending to learn whether the indigenous people can join the League of Worlds and fight an intergalactic menace; but the colonists lose contact with the League, and generations pass, with the colonists ending up facing a savage planetary threat instead.
Ursula LeGuin's Planet of Exile is the second in her loosely-connected series called The Hainish Cycle. It follows Rocannon's World, a robust high-fantasy/sci-fi mashup which I read and enjoyed recently. This novel shares many of the same themes as the colonists--branded witches by the local population--have to set aside their differences to band against shared enemies and relentless nature.
I am a big LeGuin fan and have been seeking out more of her work recently. I got this one from www.paperbackswap.com as part of a collection called Three Hainish Novels. I am sure I will read the third in this series, City of Illusions, before long.