1939–1941

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.^[9] Between 3 and 20 September she participated in laying the 'Westwall' defensive minefield in the German North Sea.^[10] Köln joined Gneisenau and nine destroyers for a sortie through the Kattegat and Skagerrak towards the Norwegian coast on 7 October. The goal was to feint a breakout into the Atlantic which would keep pressure on the British to continue convoying, and to relieve pressure from the raiding operation of the Admiral Graf Spee. The goal was also to draw the Home Fleet out of Scapa Flow and lure them into range of the Luftwaffe. The German force is found by British air reconnaissance on 8 April at the Norwegian south coast and promptly returns home, arriving on 10 October in Kiel. The Home fleet left Scapa Flow to intercept the German Force but did not find anything. A total of 148 German aircraft are deployed to attack the Home Fleet but they achieve nothing. The British launched an air attack consisting of 12 Wellington bombers, though it too failed to hit any of the German warships. The Home Fleet did not return to Scapa Flow so when U-47 penetrated the harbour defenses of Scapa Flow on 14 October to attack the Home Fleet, it found only the old battleship HMS Royal Oak lying at anchor.^[11][12]

On 20–22 November, Köln and the cruiser Leipzig escorted the battleships Gneisenau and Scharnhorst 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.^[13] On 13 December, Köln, Leipzig, and Nürnberg sortied into the North Sea under the command of vice-admiral Günther Lütjens in order to meet five destroyers returning from an minelaying operation off Newcastle and escort them home. The force was attacked at 10h45 by the British submarine HMS Salmon which scored a hit on both Leipzig and Nürnberg, and then the roles were reversed when the five destroyers had to escort the cruisers back to Germany.^[14][15]

Köln took part in Operation Weserübung, the invasion of Norway, in April 1940. She was assigned Group 3, tasked with the assault on Bergen, along with her sister Königsberg.^[16] She reached the harbor unscathed, but Königsberg was not so lucky; she was badly damaged by Norwegian coastal guns. Köln nevertheless supported the German infantry ashore with her main guns. After the port was secured, she returned to Germany, along with a pair of destroyers. In late 1940, she went into drydock for further modifications. A degaussing coil was installed, along with a helicopter landing platform on top of turret "Bruno". She thereafter served as a testbed for the Flettner Fl 282 helicopter, a task she performed until 1942.^[17]

While still conducting experiments with the FI 282 in September 1941,^[18] Köln became part of the Baltic fleet. which was assigned to guard the Baltic sea against a possible breakout of the Soviet fleet. On 23 September she left Swinemunde together with the battleship Tirpitz, the cruisers Admiral Scheer and Nürnberg, three destroyers and five torpedo boats. The fleet patrolled in the Sea of Åland and after the Soviet fleet was neutralized by the Luftwaffe in harbour, all ships returned to Gotenhafen by 29 September.^[19] As part of Operation Beowulf, Köln participated in a diversionary attack on the east coast of the Baltic island Dagö in the Gulf of Riga during the night of 12 October, so that German forces could land against a weakened defense on the south coast. On 13 October, the Soviet submarine Shch-322 tried to attack Köln, but the cruiser's escorts forced the Soviet submarine to break off the attack. The next day, Köln provided gunfire support to ground troops attacking Soviet positions on Ristna.^[20] Toward the end of 1941, she was transferred to the North Sea, and went into drydock for her last major modification. This consisted of the installation of a FuMO 21 radar set on the forward command center roof.^[21]

1942–1945

On 9 July 1942, Köln and the destroyer Z16 Friedrich Eckoldt departed Kiel to join the growing naval presence in Norway. On their way to Kristiansand the ships laid a defensive minefield in the Skagerrak. Between 14 and 15 July Köln and the destroyers Friedrich Eckoldt, Z14 Friedrich Ihn and Z24 repeated the minelaying operation. On 17 July Köln and Friedrich Eckoldt arrived in Trondheim.^[22]

On 13 September, she and the heavy cruisers Admiral Scheer and Admiral Hipper and two destroyers moved from Narvik to Altenfjord in preparation of Operation Doppelschlag, an attack on the Arctic Convoy PQ 18. While en route, the flotilla was attacked by the British submarine HMS Tigris, but the torpedoes passed behind the German ships. PQ-18 had an escort including an escort carrier, and ever since the near-torpedoing by aircraft of the British carrier HMS Victorious of the flagship of the German fleet, the battleship Tirpitz, Hitler was reluctant to risk his capital ships in an attack on a convoy and insisted the aircraft carrier be first disabled by the Luftwaffe before the ships could mount an action. As a consequence permission was not given for the flotilla to proceed with the operation, instead PQ-18 was attacked by U-boats and bombers, which sank thirteen freighters.^[23][24]

The next Arctic convoy JW 51B did not sail until end of December. As in the Arctic winter and darkness carrier escort was impossible, the Kriegsmarine was determined to attack the convoy in Operation Regenbogen, resulting in the Battle of the Barents Sea.^[25] The two available light cruisers Köln and Nürnberg did not participate in the battle as they were not considered full battle worthy.^[26] The German Navy failed to destroy JW-51B and was repulsed by the inferior British convoy escort. In the aftermath of that failed operation, a furious Hitler proclaimed that the Kriegsmarine's capital ships would be paid off and dismantled, and their guns used to reinforce the fortifications of the Atlantic Wall. Admiral Karl Dönitz persuaded Hitler to retain a battle group consisting of the battleships Tirpitz and Scharnhorst, and the heavy cruiser Lützow. But the light cruisers Köln and Leipzig and the damaged Admiral Hipper were to be decommissioned and the other cruisers were relocated to the training squadron.^[27]

Köln left Altenfjord on 23 January 1943 in company with Admiral Hipper and the destroyer Z4 Richard Beitzen to return to Germany. The three ships stopped in Narvik on 25 January, and in Trondheim from 30 January to 2 February.^[28] After resuming the voyage south, the ships searched for Norwegian blockade runners in the Skagerrak on 6 February before putting into port at Kiel on 8 February.^[29] Köln was decommissioned in Kiel on 17 February. She was sent to drydock in early 1944 for an overhaul to prepare her to return to combat duty; this was completed by 1 July. The cruiser served briefly as a training ship before escorting German merchant vessels in Norway.^[18]

On the night of 13–14 December,^[30] Köln was attacked by British bombers in Oslofjord; several near misses caused damage to her propulsion system that required repair in Germany.^[18] She then proceeded to Wilhelmshaven, where she was again attacked by Allied bombers repeatedly.^[18] On 30 March, B-24 Liberators from the Eighth Air Force attacked the harbor;^[31] Köln was hit and sank on an even keel. Since her guns remained above water, the ship was used as an artillery battery to defend the city from advancing Allied forces. She served in this capacity until the end of the war in May. She was partially dismantled in situ after the end of the war, and finally raised in 1956 for scrapping.^[32]