He was arrested during a visit to Greece in 1959, which caused a political scandal, the "Merten Affair" (Greek: Υπόθεση Μέρτεν).
After a short, two-year imprisonment, political pressure by West Germany and personal intervention by Chancellor Adenauer, led to his extradition to his homeland, where he was set free.
[1][2] According to Spiliotis,[1] the handling of the Merten case was defined by both Greek and German political and economic interests, as was the question of war crimes in postwar Greek-German relations in general.
On 28 September 1960 the West German newspapers Hamburger Echo and Der Spiegel published excerpts of Merten's deposition to the German authorities, where Merten claimed that the Greek Prime Minister Konstantinos Karamanlis was an informer during the Nazi occupation of Greece.
These statements caused a reaction by the leader of the opposition, Georgios Papandreou, and the Greek Left against Karamanlis.