Haase was a ship doctor beginning in 1927. He joined the
Nazi Party in 1933. From 1934, forward, he served on the staff of the surgery clinic of Berlin University. Upon the recommendation of
Karl Brandt, Haase began serving as Hitler's deputy
personal physician. On 1 April 1934, Haase joined the SS and on 16 June 1943 he was promoted to SS-
Obersturmbannführer. Hitler appears to have had a high opinion of him. In a telegram Hitler sent to Haase on his birthday in 1943, he stated: "Accept my heartfelt congratulations on your birthday", as reproduced in the book ''Hitler's Death: Russia's Last Great Secret from the Files of the KGB'', based on documents in Soviet archives.
April–May 1945 In late April 1945, during the last days of the fighting in the
Battle of Berlin, Haase, with
Ernst-Günther Schenck, worked to save the lives of the many wounded German soldiers and civilians in an emergency casualty station located in the large cellar of the
Reich Chancellery. During surgeries, Schenck was aided by Haase. Although Haase had much more surgical experience than Schenck, he was greatly weakened by
tuberculosis, and often had to lie down while giving verbal instruction to Schenck. The large Chancellery cellar led a further one-and-a-half meters down to an air-raid shelter known as the
Vorbunker. The
Vorbunker was connected by a stairway down to the
Führerbunker, which had become a de facto Führer Headquarters. On 23 April 1945, Hitler's personal physician
Theodor Morell and several others left Berlin by aircraft for the Obersalzberg leaving behind medications prepared for Hitler, which Haase and
Heinz Linge, Hitler's
valet, administered during Hitler's last week of life. SS physician
Ludwig Stumpfegger distributed cyanide capsules to the various military adjutants, secretaries, and staff in the bunker. Doubting the efficacy of the cyanide capsules, Hitler ordered Haase summoned to the
Führerbunker to test one on his dog
Blondi on 29 April. A cyanide capsule was crushed in the mouth of the dog, which died as a result. Hitler, in conversations with Haase during this timeframe, asked the doctor for a recommended method of suicide. Haase instructed Hitler to bite down on a cyanide capsule while shooting himself in the head. Haase remained in the
Führerbunker until
Hitler's suicide the following afternoon. Haase then returned to his work at the emergency casualty station. In the seven days they worked together, Schenck and Haase performed some "three hundred and seventy operations". Haase,
Helmut Kunz and two nurses,
Erna Flegel and Liselotte Chervinska were taken prisoner there by
Soviet Red Army troops on 2 May. On 6 May, Haase was one of those taken by the Soviet authorities to identify the bodies of the former Reich Propaganda Minister
Joseph Goebbels, his wife
Magda Goebbels and their
six children. Haase identified Goebbels' partly burned body by the metal brace Goebbels wore on his deformed right leg. ==Imprisonment and death, 1945–1950==