Legitimacy in the use of force: Opinio or Juris?

Debates about the recourse to force in international politics often conflate legality and legitimacy. The legitimate use of force is often associated with legality and illegitimacy is generally associated with illegality. But the relationship between the two concepts is not linear and recent examples such as the proposed Syrian intervention and the NATO air campaign in Kosovo show that legitimacy and legality are not necessarily cognates. This paper separates opinion from international law and argues that many commentators have inflated the role of law in determining the legitimacy of actual or proposed uses of armed force. It argues that the principle purpose of international security law is to codify norms of behaviour which states find acceptable in order to facilitate cooperation and collective action. Because laws exist to reduce risks and transaction costs between international actors, they are representations of standards which already exist and are not in themselves the final arbiter of legitimacy. Rather, legitimacy is determined by a more ethereal and dynamic combination of norms which determine what conditions actors collectively feel constitute ‘the right thing to do.’ This explains widespread advocacy for the use of force for humanitarian purposes without legal authorization under certain circumstances.