Frederick W. Kagan opens Power and Persuasion with this quote from diplomat-historian George F. Kennan:
You have no idea how much it contributes to the general politeness and pleasantness of diplomacy when you have a little quiet armed force in the background.
Kagan explains why Napolean failed and Bismarck succeeded:
Diplomacy is not simply the art of persuading others to accept a set of demands. It is the art of discerning objectives the world will accept — and the restraints on one’s own power that one must accept in turn. Peace can endure after conflict only if all the major players find it preferable to another war.