At the beginning of April 1945, a few weeks before the war ended, and struggling with a desperate shortage of skilled labour thanks to the slaughter of war, the army authorities conscripted Iwan Katz out of the concentration camp to work as an army doctor for the units in the town. It is not clear whether he had ever acquired any practical medical experience up to this point, but Medicine had been one of the subjects he had studied at university three and a half decades earlier. On 20 April 1945, he was promoted to the post of Chief Physician to the German units stationed in Mauthausen. Sources pay tribute to his role in ensuring that when U.S. troops arrived early in May 1945 the Fortress of Mauthausen was handed over without a shot being fired from either side. Katz stayed on as a physician between 6 May and 7 June 1945, attending to his fellow-survivors from the vast concentration camp, now employed not by the German military but by the American army of occupation.