Vattel dedicated the whole first part of his "Law of Nations" to the discussion of constitutionalism, giving a long and detailed explanation of what exactly was meant by states, nations and constitutions, and also what was meant by sovereignty. The first book, in short, appeared more as a treatise on constitutional philosophy, which explained what the minimum criteria should be for achieving formal equality between states and which, in this sense, laid the foundations for modern constitutionalism. At the same time, the pages which "the Law of Nations" dedicated to the internal constitution of states, to the subject of political consensus and the legitimacy of sovereignty indicated that to reach formal equality between the states it was necessary to study the mechanisms through which the political will of a nation was expressed, to study too the problem of the legitimation of sovereignty and, finally, to investigate the origins and the function of constituent power in line with a concept central in modern constitutionalism that Vattel, in 1758, was one of the first to define in this way.

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.