During a subsequent stay at Columbia University and Harvard University he completed his habilitation on “The Monroe Doctrine and its relations with American Diplomacy and Public International Law” (”Die Monroedoktrin und ihre Beziehung zur Amerikanischen Diplomatie und zum Völkerrecht“) .
He spent the winter term 1913/1914 in Paris at the Sorbonne and received his habilitation in summer 1914 from the University of Leipzig.
Between 1917 and 1919 he worked in the division for legal affairs of the German Foreign Office (Auswärtiges Amt).
In 1930 he founded the Institute of International Law at the University of Göttingen where he was the Ph.D.-supervisor of Adam von Trott zu Solz, involved in the 20 July Plot.
He published his criticism of the Nazi foreign policy in 1934 in a work titled “The crisis of inter-state thought“ („Die Krise des zwischenstaatlichen Denkens“).
After 4 years of hostilities Kraus was removed from all offices in 1937, forced to retire and banned from publishing.
However, he did not return to Göttingen before 1947 because he was defense counsel of the former president of the Reichsbank Hjalmar Schacht at the Nuremberg Trials.