'Schlieffen' was a U-boat wolfpack operation in the Atlantic against the ON.206 and ONS.20 convoys (14/22 October 1943).

The wolfpack comprised U-91, U-231, U-267, U-281, U-309, U-413, U-426, U-437, U-448, U-455, U-470, U-540, U-608, U-631, U-762, U-841, U-842 and U-844. For the loss of Oberleutnant Günther-Paul Grave’s U-470, Kapitänleutnant Lorenz Kasch’s U-540, Oberleutnant Jürgen Krüger’s U-631, Kapitänleutnant Werner Bender’s U-841 and Oberleutnant Günther Möller’s U-844, the wolfpack sank no ship.

This was the third battle of the U-boat offensive in the autumn of 1943. Despite the losses his boats had suffered in the 'Rossbach' wolfpack’s defeat and their poor performance in the battle for the for the SC.143 convoy, Grossadmiral Karl Dönitz, commander-in-chief of the German navy and still in day-to-day command of the U-boat arm via Konteradmiral Eberhard Godt, the service’s operations chief, believed that the new weapons and tactics recently introduced would now give the U-boats an operational edge, and therefore reorganised the boats still in the North Atlantic as the 'Schlieffen' wolfpack and ordered this into a patrol line to intercept anticipated westbound convoys.

These were ONS.20 and ON.206. The former departed Liverpool on 9 October, and comprised 52 merchant ships bound for Halifax, Nova Scotia, under escort of Commander H. R. Paramor’s British Escort Group B4 (frigates Bentinck, Byard, Berry, Drury, Bazely, Blackwood and Burgess together with some anti-submarine trawlers). The latter departed Liverpool on 11 October, and comprised 62 merchant ships, mainly in ballast, bound for New York and escorted by Commander R. A. Currie’s British Escort Group B6 (destroyers Fame and Vanquisher, frigate Deveron, and three corvettes). Also in the vicinity for further support was Commander P. W. Gretton’s British Escort Group B7 (destroyers Duncan and Vidette, and four corvettes).

The 'Schlieffen' wolfpack comprised a number of U-boats from the 'Rossbach' wolfpack reinforced by fresh boats newly arrived from bases in France and Germany. The wolfpack had already come under attack, while refuelling, by aircraft from the US escort carrier Card, which had damaged one boat and sunk two others, namely Korvettenkapitän Siegfried Freiherr von Forstner’s U-402 and Kapitänleutnant Joachim Deecke’s U-584. Another two boats had collided, and one of these was forced to return to base for repair. At the time of the attack on the ONS.20 and ON.206 convoys, the 'Schlieffen' wolfpack therefore comprised 14 boats.

When Allied intelligence learned of the position of the 'Schlieffen' wolfpack’s patrol line from 'Ultra' intelligence, Admiral Sir Max Horton’s Western Approaches Command decided to seek battle, increasing the defence by amalgamating the two convoys and their escorts, and sending the Escort Group B7 as reinforcement. On 15 October 1943 the convoys were sighted by Möller’s U-844, and the boats were ordered to converge on the position. By the evening of the same day a useful number of boats was in position and began the attack, achieving no success but having U-844 damaged and driven off by Duncan and Vanquisher.

Later on 16 October U-844 was attacked once more, and this time sunk by two Consolidated Liberator long-range aircraft of the RAF’s Nos 86 and 59 Squadrons. One of the aircraft was shot down, but its crew was rescued by the corvette Pink. On the same day Oberleutnant Emmo Hummerjohann’s U-964 was sunk by a Liberator of No. 86 Squadron, the survivors later being rescued by Kapitänleutnant Wolfgang Wenzel’s U-231.

Early on that day Grave’s U-470 was damaged by a Short Sunderland of the RCAF’s No. 422 Squadron. The flying boat was damaged in the attack and forced to ditch later near the ONS.20 convoy and its crew being rescued by Drury. U-470 was attacked once again, later in the day, and sunk by three Liberator aircraft (two of No. 120 Squadron and one of No. 59 Squadron). Only two of the boat’s crew were recovered by Duncan. The U-boats' only success of the day was when Kapitänleutnant Christian Reich’s U-426 sank the 6,625-ton British Essex Lance, a straggler from the ONS.20 convoy.

During the night of 16/17 October the U-boats renewed their attack, but were repelled without gaining any successes. Kasch’s U-540 was attacked and damaged by a Liberator of No. 59 Squadron, and later in the same day the U-boat was attacked again and sunk by two Liberator aircraft of Nos 59 and 120 Squadrons. Also on 17 October, Bender’s U-841 was sunk by Byard of the Escort Group B4.

During the night of 17/18 October Krüger’s shadowing U-631 was sunk by Sunflower of the Escort Group B7. At this point the convoys made a drastic alteration in course, in order to shake off any shadowers. This was successful inasmuch as Dönitz received two conflicting reports from Kapitänleutnant Heinz Hungershausen’s U-91 and Kapitänleutnant Gustav Poel’s U-413, leading to the despatch of the 'Schlieffen' wolfpack’s boats in the wrong direction. The U-boats were unable to regain contact, and Dönitz thereupon ordered the surviving boats to retire.

On 20 October, with no further attack developing, the Escort Group B7 was detached from the convoys to join the following ON.207 convoy. Responsibility for the ONS.20 convoy was handed over to warships of the Western Local Escort Force on 22 October, and arrived at Halifax without further loss on 26 October. The ON.206 convoy met its local escort group on 24 October and reached New York on 27 October.

Dönitz saw this undertaking as a setback, but remained confident that the U-boats' new weapons and tactics would achieve success. However, the loss of six U-boats for the sinking of just one ship was in reality a major German defeat, and signalled that the Germans had lost the tactical and technical initiative in the Battle of the Atlantic.