Rudolf Hess, Hitler's secretary who would assist in the writing of Mein Kampf, was a close student of Haushofer's. While Hess and Hitler were imprisoned after the Munich Beer Hall Putsch in 1923, Haushofer spent six hours visiting the two, bringing along a copy of Friedrich Ratzel's Political Geography and Clausewitz's On War.[35] After World War II, Haushofer would deny that he had taught Hitler, and claimed that the National Socialist Party perverted Hess's study of geopolitik. Hitler's biographers disagree somewhat on the extent of Haushofer's influence on Hitler: Ian Kershaw writes that "[his] influence was probably greater than the Munich professor was later prepared to acknowledge,"[36] while Joachim C. Fest says that "Hitler's version of [Haushofer's] ideas was distinctly his own."[37] Haushofer himself viewed Hitler as a half-educated man who never correctly understood the geopolitik principles explained by Hess, and saw Foreign Minister Joachim von Ribbentrop as the principal distorter of geopolitik in Hitler's mind.[38]