Many studies of the contemporary proliferation of private military companies refer to Max Weber's notion that the modern state is defined by the ‘monopoly of legitimate physical violence within a certain territory’. Weber, however, sought to define the state and not to explain why the state did, or indeed should, lay claim to the monopoly on the legitimate use of armed force. To fully understand the reasons for and the implications of the contemporary privatization of armed force in Europe and North America, one therefore needs to return to the theoretical and ideological foundations of the relationships between the state, the citizen and the soldier. What are the theoretical origins of the state monopoly on the legitimate use of violence? How did the emergence of modern democracy influence the state's exclusive right to control armed force? And what roles should the citizen and the soldier have in national and international security?

The answers to these questions have been debated over centuries. They begin with the Theory of the Social Contract between the state and its citizens for the provision of collective security. The rights and obligations of the state and the citizens under the terms of the Social Contract have been widely contested, specifically in democratic societies. During the past three centuries, two competing ideologies have been at the heart of this contest: Republicanism and Liberalism. Many theorists and politicians have shaped the debate between these two ideologies, and sometimes they have informed and even merged into one another.