This was a German naval undertaking by cruisers, destroyers and minelayers to lay a minefield as what was in effect an extension of Germany’s northward-stretching mine barrage through the eastern part of the North Sea (4/17 August 1939).
On 4 September Cobra, Roland, Erich Giese and Theodor Riedel laid 666 mines in an undertaking repeated exactly during the following day. On 8 September Cobra and Friedrich Roland laid fields of 348 and 404 mines respectively. On 13 September Cobra, Kaiser, Roland and Hansestadt Danzig, together with the destroyers Theodor Riedel, Friedrich Eckholt and Bernd von Arnim laid 960 mines. On 15 September the same force, bolstered by the destroyer Erich Steinbrinck, laid another 555 mines. On 17 September Cobra and Hansestadt Danzig laid 280 mines.