Was Emer de Vattel a constitutionalist? To what extent can the Law of Nations be considered a treatise on constitutional theory? To answer these questions, I will refer to several different case studies to illustrate and analyse the use of Vattel for the constitutional debate between the eighteenth and nineteenth century. In fact, as José M. Portillo Valdés wrote, sixty years before the Spanish constitution of June 1808, Vattel was the first who explained that only independent and sovereign entities could be labelled nations and his expression ‘free and independent’ would be used by many of the emerging republics in America and Europe.1