 Oppenhoff was an expert on Nazi law, had been legal representative for the Bishop of Aachen, Johannes Joseph van der Velden, and had defended some cases for Jewish companies.Knowing that the Gestapo was interested in him, he had taken refuge in Eupen, across the border in Belgium, in September 1944, taking his wife and three daughters with him.Following the occupation of Aachen after the Battle of Aachen, in October 1944, Allied officials wanted to appoint a non-Nazi to take over administration of the city.