Köln was a light cruiser, the third member of the Königsberg class that was operated between 1929 and March 1945, including service in World War II. She was operated by two German navies, the Reichsmarine and the Kriegsmarine. She had two sister ships, Königsberg and Karlsruhe. Köln was built by the Reichsmarinewerft in Wilhelmshaven; she was laid down in August 1926, launched in May 1928, and commissioned into the Reichsmarine on 15 January 1930. She was armed with a main battery of nine 15 cm SK C/25 (5.9-inch) guns in three triple turrets and had a top speed of 32 knots.

On 20 March 1939, Nazi Germany's foreign minister Joachim von Ribbentrop presented an oral ultimatum to Juozas Urbšys, foreign minister of Lithuania. Germany demanded that Lithuania give up the Klaipėda Region which had been detached from Germany after World War I, or the Wehrmacht would invade Lithuania and the de facto Lithuanian capital Kaunas would be bombed. The Lithuanians had been expecting the demand after years of rising tension between Lithuania and Germany, increasing pro-Nazi propaganda in the region, and continued German expansion. It was issued just five days after the Nazi occupation of Czechoslovakia. The 1924 Klaipėda Convention had guaranteed the protection of the status quo in the region, but the four signatories to that convention did not offer any material assistance. The United Kingdom and France followed a policy of appeasement, while Italy and Japan openly supported Germany, and Lithuania accepted the ultimatum on 23 March 1939. It proved to be the last territorial acquisition for Germany before World War II, producing a major downturn in Lithuania's economy and escalating pre-war tensions for Europe as a whole.