Crusader Kings II is definitely about diplomacy, but not so much about a civilization-building pacifist foreign policy type of diplomacy, as about a simulation of medieval feudal dynastic politicking.
It's not mainly about the details of warfare, but it's still pretty violent.
It's basically Game of Thrones - The Game, more so than the licencsed GoT games. It's all about incest, and intrigue, and arranged marriages, and assassinations, and legitimized bastards, and de jure land claims, and about rebellions, and deposed royal heirs taking back their birthright from usurpers, and about setting up your own council, distributing honorary titles, and cheating on your lord husband, and about putting heads on spikes.