In 1940, at the age of 58, he and his family fled Europe on the last voyage of the SS "Washington", embarking on 1 June in Lisbon. He moved to the United States, giving the prestigious "Oliver Wendell Holmes Lectures" at Harvard Law School in 1942. He was supported by Roscoe Pound for a faculty position at Harvard but opposed by Lon Fuller on the Harvard faculty before becoming a full professor at the department of political science at the University of California, Berkeley in 1945. Kelsen was defending a position of the distinction of the philosophical definition of justice as it is separable from the application of positive law. As Fuller stated his opposition, "I share the opinion of Jerome Hall, evidenced in this excellent "Readings", that jurisprudence should start with justice. I place this preference not on exhortatory grounds, but on a belief that until one has wrestled with the problem of justice one cannot truly understand the other issues of jurisprudence. Kelsen, for example, excludes justice from his studies (of practical law) because it is an 'irrational ideal' and therefore 'not subject to cognition.' The whole structure of his theory derives from that exclusion. The meaning of his theory can therefore be understood only when we have subjected to critical scrutiny its keystone of negation." Lon Fuller felt that the natural law position he was advocating against Kelsen was incompatible with Kelsen's dedication to the responsible use of positive law and the science of law. During the ensuing years, Kelsen increasingly dealt with issues of international law and international institutions such as the United Nations. In 1953-54, he was visiting Professor of International Law at the United States Naval War College.