The concept of obligations erga omnes has fascinated international lawyers for some time. It has raised high hopes about the protection of fundamental interests shared by the international community as a whole, yet its precise implications remain, at best, uncertain. My own interest in the concept goes back to a seminar, held at the Christian-Albrechts University of Kiel (Germany) in early 1998, which clearly exposed both aspects – high hopes and lack of certainty. Internships at the United Nations International Law Commission, during the final stages of its work on State responsibility (1999–2001), made me realise that obligations erga omnes not only present an intellectual challenge, but are eminently relevant to States.

This book assesses to what extent the fascinating, yet elusive, concept of obligations erga omnes has had an impact on the rules of modern international law. It is based on research undertaken at the Universities of Cambridge and Kiel. It was submitted as a PhD thesis to the University of Cambridge in late 2003, and was subsequently awarded the Yorke Prize 2005. My research in Cambridge was supervised by Professor James Crawford, to whom I am much indebted. As the International Law Commission's Special Rapporteur on the topic of State responsibility, he was in a unique position to provide expert guidance. His comments and advice proved most helpful. At the same time, I have greatly appreciated his tolerance of criticism of the Commission's work.