 His wedding to Kressner, the daughter of Colonel Karl Kressner and Hedwig Rebling, took place in Germany between these periods.After Qingdao he returned to Germany for good and taught at military academies while writing his habilitation thesis for Berlin University.During the First World War he was an officer and was wounded at the beginning of the war, which resulted in his discharge from the army; by the autumn of 1914 he was teaching at Berlin University.