Danmark decided to lay minefields in territorial waters according to a report of the American Naval Attaché at Berlin, Germany Nr. R562 dated 15 September 1939

Diary note dated Monday 4 September 1939. An item reported that the Danish cabinet decided to lay minefields in her territorial waters to prevent violating namely in the channels past Copenhagen to the southward, Kongedybet, Hollaenderdybet and in Drogden into which the two former lead.

Source

National Archives. FDR Presidential Library&Museum Roosevelt PSFC000175. War diary Naval Attaché Berlin, volume 1 (1 September 1939-26 May 1940)