This book first appeared in 2001 in Germany. It is based on my doctoral thesis written at the Max-Planck-Institute for European Legal History in Frankfurt between 1997 and 2000 in the context of a broader research project of the German Research Foundation on the history of international law and international legal scholarship in Germany in the first half of the twentieth century. As gratefully acknowledged in the German edition, many people helped me along the way, but my greatest debt is to Prof. Dr. Dr. h.c. Michael Stolleis, the director of the Max-Planck-Institute in Frankfurt and initiator of the research project, and to Prof. Dr. Eibe Riedel, who inspired, encouraged, and actively supported my work on Kelsen's international law theory.

I can now take great pleasure in the appearance of the expanded English version of the book. I have resisted the temptation to rewrite parts of the book. Instead, I have added a postscript that reflects on the potential and limitations of Kelsenian formalism in the context of recent general debates in international law. I am indebted, first and foremost, to the director of the Max-Planck-Institute for Comparative Public Law and International Law in Heidelberg, Prof. Dr. Armin von Bogdandy, for his enthusiasm, intellectual inspiration, and active support regarding the idea and execution not only of this project. My gratitude extends to Prof. Dr. Dr. h.c. Rüdiger Wolfrum, the co-director of the Institute, and to the Max-Planck-Society for very generously supporting the translation of the book.