On 27 December, Alsterufer was spotted by a fighter from an American escort carrier, then attacked by Australian, British and Canadian, Coastal Command, Sunderland flying boats but suffered little damage.

At 4:07 p.m. Liberator GR Mk V "H" of 311 (Czechoslovak) Squadron made a low-altitude attack on Alsterufer with rockets and bombs, setting the ship on fire.

Using Enigma decrypts of their positions, the German ships were bombed by US Liberators and then intercepted by the cruisers HMS Glasgow and Enterprise of Operation Stonewall.

After the supply route was closed at the start of Barbarossa and after the Japanese entry into the war, German and Italian ships were stranded in Japan and Japanese-occupied Singapore.

[3] After sailing from Japan, through the Pacific and the Indian Ocean to the Cape of Good Hope, blockade-runners kept radio silence and passed rearranging points at planned times.

The vessel went into dry dock to have the hull cleaned to increase its speed and the ship underwent sea trials, sometimes incorporating the delivery of goods to Bassens or to another Biscay port.

There were five motor vessels in Japan and it was thought that if they left at fairly frequent intervals, the Allies might be distracted by the hunt for one and let another slip through their blockades.

[7] Osorno was followed by the refrigerated cargo ship (reefer) Alsterufer (2,729 GRT, code-name Trave,Kapitän Piatek) of the Robert M. Sloman Jr. line of Hamburg, carrying 344 long tons (350 t) of tungsten, a year's worth of consumption in the German war economy.

From May 1943 decrypts of Japanese diplomatic wireless traffic revealed to the Allies that the losses of the 1942–1943 season had not deterred the Axis from making another attempt in the autumn.

[10] On 18 July the British and Portuguese reached a basing agreement for the Azores, which came into force on 8 October and which had the potential to deter the Axis from trying to run the blockade.

[12] The Ministry of Economic Warfare in London knew that the winter would be the best time for blockade-runners and photographic reconnaissance revealed that the number of German warships in the French Biscay ports had been increased.

[5] Evidence that the new round of departures from the Far East had begun was found in an Ultra decrypt of 16 November, prohibiting U-boat attacks west of a line in the south Atlantic.

[13] Little was revealed by OP-20-G, the US Navy (USN) code-breaking organisation, until 26 November, that on the day before the U-boat restrictions ("Kammerarrest") in the south Atlantic had been imposed further north on 1 December.

[14][b] On 26 November the Italian ship Pietro Orseolo sailed from Bordeaux to Concarneau on the south Brittany coast and was attacked by aircraft from Coastal Command on 1 December to no effect.

[14][c] The Admiralty signalled the importance given to preventing the arrival of blockade-runners on 12 December and the Royal Navy light cruiser HMS Glasgow left on patrol from the Azores.

The Royal New Zealand Navy (RNZN) light cruiser HMNZS Gambia, recently refitted, arrived at Plymouth from Scotland on 5 December 1943.

Gambia was to join Glasgow and Enterprise under the command of the Commander-in-Chief Plymouth (Admiral Ralph Leatham) for operations against blockade runners.

[20][13] On 18 December Osorno passed itself off as the British Landsman en route from Cape Town to Liverpool to a Sunderland flying-boat that investigated it.

[20] On 23 December, a F4F fighter from Card sighted a ship about 500 nmi (930 km; 580 mi) south-west of Ushant; Osorno failed to give the right answer to the challenge, despite flying the Red Ensign.

Card had to close its flight-deck after accidents then the Task Group was distracted by the attacks of Wolfpack Borkum which had the benefit of aircraft flying from land bases.

At noon on a cloudy day, when about 450 nmi (520 mi; 830 km) west of the French coast, the lookouts on Osorno spotted destroyers with characteristic German funnel caps.

[26] Bomber Command sent five Stirling mine-layers on the night of 29/30 December and the waters around the ship were mined to obstruct the unloading of its cargo of rubber but the Germans got most of it ashore.

[27] Alsterufer passed the South Atlantic narrows undetected and crossed the US–Gibraltar route on 20 December, not far from TG.21.16, which included the carrier USS Block Island and four destroyers.

Torpedobootflottille), less ZH 1, which had engine-trouble, sailed into the Bay of Biscay again on Unternehmen Trave (Operation Beam) on 26 December to meet Alsterufer and escort it into the Gironde.

[29] Alsterufer was spotted at 10:15 a.m. on 27 December about 500 nmi (580 mi; 930 km) north-west of Cape Finisterre and kept Sunderland "T" of 201 Squadron at a distance with anti-aircraft fire.

If I released our bombs we just couldn't miss, but their forward speed, being the same as that of our aircraft, would have resulted in explosions directly under us, with the consequent dire damage to ourselves.

[33] Alsterufer had opened fire with anti-aircraft guns and parachute-and-cable rockets, hitting the Liberator's starboard outer engine but the aircraft returned to base at RAF Beaulieu in England.

[29] Naval Group West did not find out about the loss of Alsterufer until morning on 28 December and cancelled Unternehmen Bernau, ordering the ships to return.

The engagement also finally made the Admiralty admit that the German Type 1936A destroyers (Narvik-class to the British) carried 6 in (150 mm) guns.