In 1939 Krull left Erlangen to take up a chair at Bonn. However, his career was disrupted by the Second World War which began shortly after Krull was appointed to the University of Bonn.
Sa 1939 Krull kaliwa Erlangen na tumagal ng isang upuan sa Bonn. Subalit, ang kanyang karera ay disrupted ng Ikalawang Pandaigdig na nagsimula ilang sandali matapos Krull ay hihirangin upang angkinin ang University of Bonn.