He went to Austria in 1938 to prepare the ground for Anschluss. He served as Secretary at the German Embassy in Vienna in 1938, Reich Commissioner in Austria from March to June 1938, then Reich Commissioner in Slovakia in 1939, and finally Reich Commissioner in Danzig in August 1939. Keppler became Secretary of State with special duties in the Foreign Office during World War II, during which he administered SS confiscated industries in Poland and Russia. On 30 January 1942 he became an honorary "Obergruppenführer" (Lieutenant General) of the SS.