In the final days of August 1939, "Köln" was stationed in the western Baltic to prevent Polish vessels from fleeing after the planned German invasion of Poland on 1 September; she was unsuccessful in this task. She thereafter joined her sister ships in laying a series of defensive minefields."Köln" joined "Gneisenau" and nine destroyers for a sortie into the North Sea on 7–9 October. The goal was to draw units of the Royal Navy over a U-boat line and into range of the Luftwaffe, though it failed on both counts. The British launched an air attack consisting of 12 Wellington bombers, though it too failed to hit any of the German warships. On 20–22 November, "Köln" and the cruiser escorted the battleships "Gneisenau" and on the first leg of their sortie into the North Atlantic. On the 22nd, "Köln" and "Leipzig" were detached to join an unsuccessful patrol for Allied merchant ships in the Skagerrak along with "Deutschland" and three torpedo boats. The patrol lasted until 25 November, and failed to locate any Allied freighters. On 13 December, "Köln", "Leipzig", and covered the return of several destroyers that had laid an offensive minefield off Newcastle.