For 350 years, intervention has been the 'sweet enemy" of the European states system, as it now is of the wider international system. A system of states based upon territorial integrity and political sovereignty must view the possibility of legitimate intervention as, at least potentially, one of the most fundamental challenges possible to its ruling feature. Yet in the modern history of the theory and practice of international relations the twin notions of sovereignty and legitimate intervention are, like England and France in the sixteenth century, bound together in mutual dependence and reciprocity even as they sit, precariously, on opposite sides of a conceptual chasm. In this chapter, the question, "When is it legitimate for one state to intervene in the affairs of another?" will be approached from a perspective which tries to formulate and revitalise certain aspects of the classical traditions of international relations discourse. In the process a different light will be cast on the nature of the relationship between the twin pillars of the contemporary international system: sovereignty and intervention.