[2] When the National Socialist Party came to power in 1933, Hardenberg refused to join any of the party-sponsored organizations and was removed from all his positions.
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.
Had the coup d'état succeeded according to the initial plan, Hardenberg would have taken the position of prime minister of Berlin-Brandenburg.
But his estate was nationalized by the Communist authorities, and he fled with his family to West-Germany and lived in Kronberg im Taunus, Hesse.