'Bernau' was a German undertaking in the Bay of Biscay to provide escorts, in the form of U-boats of the 'Borkum' (i) wolfpack, for the 6,951-ton blockade runner Osorno, inward bound from Kobe in Japan to Bordeaux on the western coast of German-occupied France with a cargo of rubber and important minerals (24/26 December 1943).

The Allied response to the German undertaking, which also included the blockade runner Alsterufer, escorted by German destroyers, was 'Stonewall'. The New Zealand light cruiser Gambia joined the Allied operation in December 1943, and operated from Horta in the Azores islands group, with the British light cruiser Glasgow, patrolling an area to the north of the islands. On 23 December aircraft from the US escort carrier Card spotted a suspected blockade runner, and Gambia, Glasgow and another British light cruiser, Enterprise, formed a cordon to intercept. Aircraft attacked the flotilla escorting Osorno, and reported a hit and a near-miss on the blockade runner.

At 12.00 on 24 December the destroyers of Kapitän Hans Erdmenger’s 8^th Zerstörer-Flottille (Z-23, Z-24, Z-27, Z-32, Z-37 and ex-Dutch ZH-1), and the torpedo boats of Korvettenkapitän Franz Kohlauf’s 4^th Torpedoboots-Flottille (T-22, T-23, T-24, T-25, T-26 and T-27) steamed out of the Gironde river estuary in the west of German-occupied France to meet Osorno in 'Bernau'. Although from dawn on 25 December Short Sunderland four-engined flying boats of the RAF’s No. 201 Squadron, RCAF’s No. 422 Squadron and RAAF’s No. 461 Squadron were in contact, Erdmenger’s ships were able to meet Osorno after she had shot down a flying boat. Attempted attacks by aircraft of Air Vice Marshal B. E. Baker’s No. 19 Group of Air Marshal Sir John Slessor’s RAF Coastal Command were beaten off by Junkers Ju 88 twin-engined long-range fighters ordered into the air by General Ulrich Kessler, the Fliegerführer 'Atlantik', and by the warships' anti-aircraft fire. Entering the Gironde estuary on 26 December, Osorno struck the wreck of Sperrbrecher 21, damaging her hull, and had to be run aground to save her cargo of 3,882 tons of rubber, 1,797 tons of tin and 177 tons of wolfram, which were unloaded on the shore.