 From 1939 Grewe lectured at the "Hochschule für Politik" (School of Politics) in Berlin, which, in 1940, was incorporated into the new "Auslandswissenschaftliche Fakultät" (Faculty of Foreign Studies) at the Friedrich Wilhelm University of Berlin, where he served as lecturer in "Legal Foundations of Foreign Policy".In 1941 Grewe was awarded his habilitation on the basis of work that was later to become part of his book "Epochen der Völkerrechtsgeschichte", and he lectured at the Faculty of Foreign Studies and the Faculty of Law (in Berlin).In 1942 he was appointed extraordinary professor at the Faculty of Foreign Studies, where he taught "Legal Foundations of Foreign Policy" and "Policy of International Law" ("Völkerrechtspolitik").