Between 1939 and 1943 he was based at the Foreign Office in Berlin's government quarter. He was involved in the diplomatic subterfuge preceding the invasion of Denmark in 1940, and carried instructions to the embassy, together with a general's uniform for Kurt Himer, in his diplomatic bag to Copenhagen on 9 April 1940, the day of the invasion. The information Schlitter delivered equipped Himer and the embassy staffs in Copenhagen and Oslo to know what was going on.