Special Circumstances is a reference to the "totally not a military but we happen have found this incredible amount of fire power just lying around" non-military section of the Culture culture.
It's Bank's wink towards that while you might have an enlightened view of universal peace other parts of the universe would quite like you in pieces.
Essentially the Culture's attitude to violence is that it should be used as a last resort, so named because once resorted to it lasts as an example.