In 1939 he was called for military duty as major in the military reserve force. In 1940 he was assigned to Army Group B where he met Major General Henning von Tresckow and became involved in the resistance against Hitler. In 1942 he was transferred to Berlin where he came into contact with colonel Claus Schenk Graf von Stauffenberg and began organizing for the plot for the assassination of Hitler. Many of the meetings of the conspirators took place at the Neuhardenberg mansion, which was located at 70 km from Berlin and was not under police supervision. Had the coup d'état succeeded according to the initial plan, Hardenberg would have taken the position of prime minister of Berlin-Brandenburg.