With the electoral success of Hitler's political party, he also became a member of the Reichstag and of the Staatsrat of Prussia. Philipp played an important role in the consolidation of National Socialist rule in Germany. He introduced other aristocrats to NSDAP officials and, as son-in-law of the king of Italy, was a frequent go-between for Hitler and Benito Mussolini. He also acted as an art agent for Hitler in Italy.. On March 11th 1938 he delivered Hitler's letter of reassurance to Mussolini prior to Hitler's marching into Austria, and received a gushing call from Hitler upon the news of Mussolini's acquiescence. As Governor of Hesse-Kassel, Philipp was complicit in the Aktion T4 euthanasia programme. In February 1941, Philipp signed the contract placing the sanitarium of Hadamar Clinic at the disposal of the Reich Interior Ministry. Over 10,000 mentally ill people were killed there. In 1946, Philipp was charged with murder, but the charges were later dropped. As the war progressed, the attitude of the National Socialist authorities towards members of the German princely houses changed. While at first they had been happy to use the historic family names to bolster popular support, they now decided to distance themselves even from those princes who had supported them. In late April 1943, Philipp was ordered to report to Hitler's headquarters, where he stayed for most of the next four months. In May 1943, Hitler issued the "Decree Concerning Internationally Connected Men" declaring that princes could not hold positions in the party, state, or armed forces. The arrest of Mussolini by Philipp's father-in-law King Victor Emanuel in July 1943 made Phillip's position even more difficult. Hitler believed that Philipp and his family were complicit in Mussolini's downfall. On 8 September 1943, Philipp was arrested. He was stripped of his membership in the party and dismissed from the Luftwaffe. On 25 January 1944, his political disgrace became public when he was dismissed from his office as Governor of Hesse-Nassau. In September 1943, Philipp was sent to Flossenbürg concentration camp. He was placed in solitary confinement and was not permitted any contact with the outside world. He was, however, granted certain privileges: wearing civilian clothes and eating the same food as the guards. Philipp's wife Mafalda was arrested and placed under military custody in Rome. She was sent to Munich and Berlin for questioning and eventually to Buchenwald concentration camp where she was housed next to an armaments factory.