In 1934, at the age of 52, he published the first edition of "Reine Rechtslehre" ("Pure Theory of Law"). While in Geneva he became more deeply interested in international law. This interest in international law in Kelsen was in reaction largely to the Kellogg–Briand Pact in 1929 and his negative reaction to the vast idealism he saw represented in its pages, along with the lack of the recognition of sanctions for the illicit actions of belligerent states. Kelsen had come to endorse strongly the sanction-delict theory of law which he saw as substantially under-represented in the Kellogg–Briand Pact. In 1936–1938 he was briefly professor at the German University in Prague before returning to Geneva where he remained until 1940. His interest in international law would become especially focused in Kelsen's writings on international war crimes which he would redouble his efforts on behalf of after his departure to the United States.