He was married to the daughter of Nikolaus von Falkenhorst, who planned the German invasion of Norway and Denmark during World War II.

After his recovery, he rose to the rank of Generalmajor, and served on the army General Staff in Adolf Hitler's headquarters.

Dethleffsen was arrested on 23 May 1945, and was held until March 1948 in an American Prisoner of War Camp.

He was originally held in Luxembourg with Hermann Göring, Joachim von Ribbentrop, and others.

On his release, Dethleffsen became executive secretary of the Wirtschaftspolitische Gesellschaft von 1947 (Society of 1947 for Economic Policy.)