If the point of constitutionalism is to define the legal framework within which collective self-government can legitimately take place, constitutionalism has to take a cosmopolitan turn. Contrary to widely made implicit assumptions in constitutional theory and practice, national constitutional legitimacy is not self-standing. Whether a national constitution and the political practices authorized by it are legitimate does not depend only on the appropriate democratic quality and rights respecting nature of domestic legal practices. Instead, national constitutional legitimacy depends, in part, on how the national constitution is integrated into and relates to the wider legal and political world. The drawing of state boundaries and the pursuit of national policies generate justice sensitive externalities that national law, no matter how democratic, can not claim legitimate authority to assess. It is the point and purpose of international law to authoritatively address problems of justive-sensitive externalities of state policies. International law seeks to help create the conditions and define the domain over which states can legitimately claim sovereignty. States have a standing duty to help create and sustain an international legal system that is equipped to fulfill that function. Only a cosmopolitan state -- a state that incorporates and reflects in its constitutional structure and foreign policy the global legitimacy conditions for claims to sovereignty -- is a legitimate state.