It is no great surprise that in the contemporary world the use of deadly force by a political grouping or nation-state or on behalf of international society should raise troubling questions of legitimacy. The problem appears to be massively overdetermined and the intellectual challenge is to bring some order to a confused and confusing debate; to distinguish between short-term problems and deep-rooted changes in both understandings of legitimacy and patterns in the use of force; and try and identify where there might be scope for narrowing the very deep disagreements that have come to surround this question.