'Amsel II' was a U-boat wolfpack operation in the Atlantic (4/6 May 1943).

The wolfpack comprised U-107, U-223, U-266, U-377, U-383 and U-634, and for the loss of none of its own number sank three ships (12,012 tons) in attacks on the ONS.5 convoy.

After their successful convoy battles of March 1943, most of the U-boats were near the limits of their patrol endurances, and had therefore to return to base for supplies and repairs. As as result, the smaller number of boats at sea and the reinforcement of the escort forces combined to trim the German success rate during April by some 50%. At the end of this month, though, most of the U-boats based in western France returned to operational status and, with the arrival in the North Atlantic directly from Germany of U-boats that had only recently been completed and commissioned, the U-boat strength along the North Atlantic convoy routes attained a new peak.

Also at its peak at the time was the continuing intelligence war between the Germans and the Western Allies. Each side had broken the other’s codes, but the Allied 'Ultra' code-breakers were generally able to decrypt the German messages more quickly, and this allowed the Allied naval command authorities to route convoys around U-boat packs, thereby stymying the codebreakers of the B-Dienst, whose slower decrypts of Allied messages provided the U-boat arm with information that was slightly out of date.

On 25 April, however, there was a temporarily break on the UK’s code-breaking success after the Germans unexpectedly changed their codes, and on the following day the 'Star' wolfpack was formed to intercept an expected westbound convoy. This German movement was not detected in time in order to a re-routing of the ONS.5 convoy of 42 ships supported by Commander P. W. Gretton’s British Escort Group B7 (destroyers Duncan and Vidette, frigate Tay, corvettes Loosestrife, Pink, Snowflake and Sunflower, and anti-submarine trawlers Northern Gem and Northern Spray).

The ONS.5 convoy was routed on a northerly course to extend the period in which it would benefit from cover by aircraft operating from Iceland. Recent convoy battles had taken place farther to the south, and it was hoped that this change would keep the ONS.5 convoy clear of the wolfpacks. On 24 April the air escort sank Oberleutnant Dietrich von Carlewitz’s U-710, which chanced on the convoy on its way from Germany into the Atlantic. On 28 April the convoy ran into the German patrol line and was reported by Kapitänleutnant Ernst von Witzendorff’s U-650. The boat’s contact reports and the transmissions of other boats (including U-386 and U-378) trying to close with the convoy were detected by the escorts' HF/DF equipment, and it thus became clear to Gretton that a major attack was imminent.

The convoy was therefore ordered to take evasive action, and the escorts made sweeps toward the HF/DF bearings. Only four other boats managed to come up during the evening, and their attacks in the night were unsuccessful. Only Fregattenkapitän Ottoheinrich Junker’s U-532 was able to launch but missed, and Oberleutnant Hans-Albrecht Kandler’s U-386 and U-532 were severely damaged by the escorts and had to return to base. On 29 April Kapitänleutnant Wilhelm von Mässenhausen’s U-258 sank the 6,198-ton US McKeesport in a submerged daylight attack, but Kapitänleutnant Georg von Rabenau’s U-528 was damaged by the air escort and had to return to base.

The Allies responded to these initial attacks by diverting to the ONS.5 convoy’s escort the destroyer Oribi from the SC.127 convoy and Captain J. A. McCoy’s 3^rd Support Group (destroyers Offa, Impulsive, Penn and Panther) from St John’s, Newfoundland.

During the following days the weather became steadily worse and only Oberleutnant Werner Happe’s U-192 managed to make an attack, which was unsuccessful, before the U-boats lost contact. Though the storm was useful in this respect for the convoy, it also caused problems inasmuch as the convoy became scattered and the escorts could not refuel. Eventually six ships straggled and another six were re-routed apart from the main body under escort by Pink.

Meanwhile the B-Dienst had located the eastbound SC.128 convoy, and this now became the primary target for a grouping of nearly all the boats that were available from the 'Star' and new 'Specht' wolfpacks as well as the newly arrived boats that established the 'Fink' wolfpack on a new patrol line across the convoy’s expected path. Fresh boats coming from bases in German-occupied France created the 'Amsel' wolfpack that was subdivided in four smaller groups. Although the SC.128 convoy was sighted and reported by U-628 on 1 May, the convoy was informed of the wolfpacks' location by 'Ultra' decrypts and managed to avoid all of them. Aircraft were also despatched to sweep the U-boats' operating areas, and a Boeing Canada Canso flying boat damaged Kapitänleutnant Heinrich Brodda’s U-209, which was on passage to a patrol line and disappeared several days later. Korvettenkapitän Heinrich Heinsohn’s U-438 was also damaged by air attack.

During the evening of this day, Kapitänleutnant Heinrich Hasenschar’s U-628 sighted and reported not the eastbound and expected SC.129 convoy but rather the westbound ONS.5 convoy which, as a result of the adverse weather, had been able to make only 20 miles (32 km) per day and now ran into the rear of the German patrol line.

Grossadmiral Karl Dönitz, still in day-to-day command of the U-boat arm despite being the commander-in-chief of the German navy, immediately ordered the 'Fink', 'Amsel I' and 'Amsel II' wolfpacks to close on the ONS.5 convoy, which has been joined by Oribi and the 3^rd Escort Group, and had managed to regain most of its stragglers. Only four ships were far to the rear, where they were still being escorted by Pink. On 3 May the destroyers Duncan, Impulsive, Panther and Penn, together with the trawler Northern Gem, left the convoy as a result of their shortage of fuel.

From the increasing number of HF/DF interceptions in the area of the convoy, it was now clear that the U-boats were closing in, and as a result Captain G. N. Brewer’s 1^st Support Group (sloops Pelican and Sennen, and frigates Jed, Spey and Wear) was despatched from St John’s as reinforcement.

The attacks began even before the fall of night when Kapitänleutnant Ulrich Folkers’s U-125 sank the straggling 4,737-ton British Lorient. As night fell, the escorts initially managed to keep the U-boats at a distance. Kapitänleutnant Paul-Friedrich Otto’s U-270 and Oberleutnant Claus-Peter Carlsen’s U-732 were depth-charged and damaged by Oribi and Snowflake, and by Vidette respectively, and both boats had to make for port. Kapitänleutnant Hans-Jürgen Auffermann’s U-514 was damaged by Vidette and had to pull back from the battle, but was able to make repairs and resume its patrol after some days.

But finally the defence collapsed and the U-boats were able to get through: Oberleutnant Günter Gretschel’s U-707 attacked from the front, dived under the convoy and then surfaced to its rear, where it sank the straggling 4,635-ton British North Britain. Then Hasenschar’s U-628 managed to penetrate into the convoy through a gap in the escort, launched five torpedoes at separate targets, but sank only the 5,081-ton British Harbury. Only minutes later Kapitänleutnant Hartwig Looks’s U-264 also launched five torpedoes with greater success to sink the 5,561-ton US West Maximus and 4,586-ton British Harperley with two torpedoes each. Finally Kapitänleutnant Rolf Manke’s U-358 fired three torpedoes and sank the 2,864-ton British Bristol City and 5,212-ton British Wentworth.

Sensing the possibility of a major victory after six weeks of poor results, Dönitz urged his U-boat captains to press home their attacks whenever the opportunity offered. But the escorts were now operating so effectively during night that submerged daylight attacks seemed more promising. This kept the escorts very busy, to the extent that most were soon short of depth charges or/and fuel. The 5,507-ton British Dolius was sunk by U-638, which was itself then sunk by Sunflower. Toward evening Kapitänleutnant Ralf von Jessen’s U-266 fired four torpedoes and sank the 1,570-ton Norwegian Bonde, 5,306-ton British Gharinda, and 5,136-ton British Selvistan, but was then damaged by Offa.

Some of the U-boats managed to reach the group escorted by Pink, and while the corvette was depth-charging Manke’s U-358, damaging it so severely that the boat had to turn base for repairs, Kapitänleutnant Joachim Deecke’s U-584 got to the unprotected group and sank the 5,565-ton US West Madaket.

Toward evening a Consolidated B-24 Liberator long-range patrol bomber appeared overhead and provided cover for as long as the light lasted, but could not break up the concentration of U-boats round the convoy. Before dusk Tay alone had already sighted seven U-boats, and at least 15 boats were in contact.

Then the convoy reached a thick bank of fog and the whole situation changed. While the U-boats had expected to overrun the escorts by using their superiority in numbers, the escorts now had the advantage in being able detect the U-boats in both an easy and a timely fashion with their radar, which in the case of all the escorts was the new Type 271M equipment operating on wavelengths too short to be detected by the U-boats' Metox radar detectors.

Between the evening and the following morning, the boats made some 25 attacks that were all repulsed in timely fashion by the escorts. Shortly before 24.00 Vidette located Kapitänleutnant Herbert Neckel’s U-531 on radar and sank the boat with two depth-charge patterns as it tried to make a late crash-dive. Vidette went on to chase Gretschel’s U-707 and other radar contacts, but without result. Then toward morning a 'Hedgehog' attack on a sonar contact sank Oberleutnant Werner Winkler’s U-630. Loosestrife initially made an unsuccessful depth-charge attack on Kapitänleutnant Günther Heydemann’s U-575 and then detected Happe’s U-192 on radar. Just as the corvette appeared out of the fog at a distance of 550 yards (500 m), U-192 launched two torpedoes at the warship, but both missed and the U-boat was then destroyed on the surface with a shallow-set depth-charge pattern. Snowflake attacked and drove off Kapitänleutnant Harald Gelhaus’s U-107 with depth charges, and got another four radar contacts: while engaging some of these with gunfire, the corvette forced the crew of Folkers’s U-125 to scuttle their boat after it had been rammed and immobilised by Oribi.

Offa made five attacks before midnight, but without inflicting any damage, and only toward morning managed to inflict slight damage on Kapitänleutnant Karl-Jürg Wächter’s U-223 with gunfire and depth charges.

Sunflower made numerous attacks on the radar contacts round her, was targeted on two occasions by the U-boats, but had to wait until morning to see some result for her efforts: Kapitänleutnant Helmut Hennigs U-533 was rammed but escaped with limited damage and was able to resume its patrol after repairs.

When the ships of the 1^st Support Group arrived in the morning, they dispersed round the convoy as a distant screen before joining the close escort and driving away the boats still waiting their opportunity in the area. Pelican followed a radar contact and, as a result of the fog, was able to close to within 330 yards (300 m) before being spotted by Heinsohn’s U-438, which crash-dived too late and was destroyed by two patterns of depth charges.

Sennen went to support Pink, and on her way attacked von Witzendorff’s U-650 and Heydemann’s U-575 with depth charges and 'Hedgehog' bombs, but achieved no success.

Spey assumed position behind the convoy and drove off Oberleutnant Eberhard Dahlhaus’s U-634, which was hit twice by gunfire but escaped.

Toward morning the full scale of the disaster to his U-boats was finally appreciated by Dönitz, who immediately called off the operation. The boats were regrouped, but only half of them were still capable of further combat. The Germans laid the blame for their failure on the fog and the general fact that most of their successes had been obtained in the first night of the battle when the U-boats had the advantage of surprise. More worrying, however, was the fact that a very large concentration of U-boats had been defeated by the actions of surface escorts alone. It is also remarkable that only half of the boats actually made contact with the convoy, and there was also a certain reluctance to press home attacks. The convoy lost 13 ships, and this was the last occasion on which a single convoy lost a large number of ships. In exchange for these 13 ships, the Germans lost six U-boats, to which could be added von Carlewitz’s U-710 to an air attack and Brodda’s U-209 possibly in a diving accident after suffering damage in an air attack, and another seven U-boats had to abort after sustaining severe battle damage.

The battle of the ONS.5 convoy is generally and rightly regarded as the decisive turning point in the Battle of the Atlantic. In the following three weeks the Germans were unable to attack any other convoy, but nonetheless continued to suffer severe losses at the rate of one U-boat per day to the end of May. On 23 May Dönitz ordered a halt to all U-boat operations against convoys and recalled the boats from the North Atlantic convoy routes until new tactics and weapons were available. Two of the factors Dönitz now had to take into account as he pondered the way forward for the U-boat arm in the North Atlantic were the tactical futility of seeking to assemble and control large numbers of U-boats against a single convoy (two of the U-boats had collided and foundered), and the strength of the escort forces that the Allies could now assemble, in terms of the numbers of ships and the capabilities of these ships in terms of their sensors and weapons), as well as the skill with which they now fought.