He also joined the Abwehr's Foreign Department at the Reich Defence Ministry from 1929 to 1931 and from 1935 onwards (with a break to command two ships and for a role at the Naval Academy Mürwik).

He was in command of German forces there throughout its long siege in 1944-45 (being promoted to Vizeadmiral (vice admiral) on 30 September 1944), and signed its unconditional surrender at the end of the war.

Against a background of accelerating strategic collapse by the Germans, he maintained a level of morale and effectiveness in his troops resembling that of the much more capable pre-1943 Wehrmacht.

Handbill from Allied aircraft thrown over the positions of German troops in Dunkirk in the month of April or in the very first days of May 1945.

Zur Besprechung der Übergabe wird Admiral Frisius oder ein beglaubigter Vertreter durch die alliierten Linien gelassen.