Hail the pioneer! Jean Monnet devised a method, both cautious and bold, pragmatic and ideological, aimed at integrating nations that had often been at war with one another, many of which were proud of their past and all of which had emerged after 1945 in a world where Western Europe was no longer the core of international affairs but a dependent of one superpower and a potential victim of the other. Political scientists needed to grapple with an approach that aimed at transcending the nation-state yet was quite different from classical federalism because it was functionally piecemeal and institutionally more technocratic than democratic.
Haas was the first theorist of the Monnet method. In this (early) study of the Coal and Steel Community, he introduced all the concepts that the huge subsequent literature on European integration has used and argued about ever since. Even those who disagree with his theory (especially with his notion of "spillover," which suggested that integration in one functional area would almost necessarily lead to integration in others) on empirical or normative grounds owe him a huge debt -- especially as Haas himself did not remain stuck in his ideas, but candidly acknowledged that the development of what is now the European Union did not always fit his original theory. He built the solid foundations of what is now a huge theoretical edifice.