Excession deals with this. It involves the Culture and "The Affront" which is a spacefaring but savage civilization that some people in the Culture dislike. Pla

Excession deals with this. It involves the Culture and "The Affront" which is a spacefaring but savage civilization that some people in the Culture dislike. Player of Games is a similar story about a civilization that the Culture dislikes. Those are my two favorite Culture books among my favorite books in general. The Wikipedia articles about the books goes into spoiler-heavy details about the Culture's interaction with those civilizations.