Köln

Köln was ordered as "Cruiser D" under the contract name Ersatz Arcona, replacing the old cruiser Arcona. Her keel was laid at the Reichsmarinewerft in Wilhelmshaven on 7 August 1926, she was launched on 23 May 1928, and she commissioned into the Reichsmarine on 15 January 1930 — the last of her three-ship class to be completed, after Königsberg and Karlsruhe. She was 174 m long overall with a 15.2 m beam, displaced 7,700 long tons at full load, and carried 21 officers and 493 enlisted men.

Her main battery was nine 15 cm SK C/25 guns in three triple turrets, one forward and two superfiring aft, with the after turrets offset to widen their arcs of fire and 120 shells per gun. She carried four triple torpedo tube mounts amidships with twenty-four 50 cm torpedoes, and could lay 120 mines. Propulsion was a mixed arrangement of four steam turbines fed by six oil-fired water-tube boilers and a pair of 10-cylinder four-stroke diesels, giving 32 knots and about 5,700 nautical miles at 19 knots. Armour was light: a 50 mm belt, a 40 mm deck, 100 mm on the conning tower.

After a year of trials in the Baltic she went into the Atlantic in early 1932 for more extensive testing, then took aboard her first class of naval cadets for a world cruise that lasted a full year from late 1932. She called in the Atlantic, Pacific and Indian Oceans and the Mediterranean; in Australia the tour took in Adelaide, Melbourne, Sydney and Hobart, with the crew playing publicised football matches against local sides including a Royal Australian Navy team in Sydney.

An aircraft catapult and cranes for floatplanes were fitted in 1935. She served as a training ship until early 1936, moved to fishery protection, and later that year joined the non-intervention patrols off Spain. After Republican bombers attacked the heavy cruiser Deutschland, Köln carried her wounded crewmen home to Germany, then made four further patrols off the Spanish coast before returning to fishery protection in the North Sea in 1938. In March 1939 she sailed to Memel in connection with Germany's annexation of the Memelland district from Lithuania.

In the first weeks of the war she helped lay the Westwall defensive minefield in the German North Sea, and on 7 October 1939 joined Gneisenau and nine destroyers in a sortie towards the Norwegian coast intended to feint an Atlantic breakout and draw the Home Fleet within reach of the Luftwaffe. British air reconnaissance found the force on the 8th and it returned home. On 13 December she sortied with Leipzig and Nürnberg under Vice-Admiral Lütjens to meet destroyers returning from minelaying off Newcastle; the British submarine Salmon hit both of her consorts, and the destroyers ended up escorting the cruisers home.

For Operation Weserübung in April 1940 she was assigned to Group 3, the assault on Bergen, alongside her sister Königsberg. Köln reached the harbour unscathed and supported the German infantry ashore with her main guns; Königsberg was badly damaged by Norwegian coastal guns. Köln was the only member of her class to survive the operation. In late 1940 she was drydocked for a degaussing coil and a helicopter landing platform on top of turret Bruno, and served until 1942 as the testbed for the Flettner Fl 282.

In September 1941 she joined the Baltic fleet guarding against a Soviet breakout, sailing from Swinemünde with Tirpitz, Admiral Scheer, Nürnberg and their escorts. During Operation Beowulf she made a diversionary attack on the east coast of Dagö on the night of 12 October so that German troops could land against a weakened defence on the south shore. The Soviet submarine Shch-322 tried to attack her on the 13th and was driven off by her escorts, and the next day she shelled Soviet positions on Ristna in support of ground troops.

Sent north again in July 1942, she laid defensive minefields in the Skagerrak and reached Trondheim on the 17th. On 13 September she moved with Admiral Scheer and Admiral Hipper from Narvik to Altafjord for Operation Doppelschlag against convoy PQ 18, surviving a torpedo attack by the submarine Tigris on the way. Permission to attack was withheld because the convoy had an escort carrier and Hitler would not risk heavy ships until the Luftwaffe had disabled it; PQ 18 was left to U-boats and bombers, which sank thirteen freighters.

She and Nürnberg were held back from Operation Regenbogen against JW 51B in December as not fully battle worthy. The resulting Battle of the Barents Sea, in which an inferior British escort repulsed the German force, brought Hitler's fury down on the surface fleet. Dönitz saved a battle group of Tirpitz, Scharnhorst and Lützow, but Köln, Leipzig and the damaged Admiral Hipper were to go. She left Altafjord on 23 January 1943 and was decommissioned at Kiel on 17 February.

An overhaul in early 1944 brought her back for training duty and then escort work for German merchant ships in Norway and further minelaying. British bombers near-missed her in Oslofjord on the night of 13–14 December 1944, damaging her propulsion badly enough to need repair in Germany. At Wilhelmshaven she was bombed repeatedly, and on 30 March 1945 B-24 Liberators of the Eighth Air Force hit and sank her. She settled on an even keel with her gun turrets still above water, and was used as an artillery battery defending the city until the war ended in May. She was partly dismantled where she lay and finally raised in 1956 for scrapping.