Editing Battle of the Atlantic (section)

==Climax of the campaign (March–May 1943, "Black May")==
On 10 March 1943, the Germans added a refinement to the U-boat Enigma key, which blinded the Allied codebreakers at Bletchley Park for 9 days.<ref>Erskine, Ralph; Smith, Michael (2011). ''The Bletchley Park Codebreakers''. London: Biteback Publishing, 2011.</ref> That month saw the battles of convoys [[UG convoys|UGS 6]], [[Convoy HX 228|HX 228]], [[Convoy SC 121|SC 121]], [[Convoys HX 229/SC 122|SC 122 and HX 229]]. One hundred and twenty ships were sunk worldwide, 82&nbsp;ships of 476,000&nbsp;tons in the Atlantic, while 12&nbsp;U-boats were destroyed.

The supply situation in Britain was such that there was talk of being unable to continue the war, with supplies of fuel being particularly low. The situation was so bad that the British considered abandoning convoys entirely.<ref>Milner, ''North Atlantic Run''? van der Vat, ''The Atlantic Campaign''?</ref> The next two months saw a complete reversal of fortunes.

In April, losses of U-boats increased and their kills fell significantly. Only 39 ships of 235,000&nbsp;tons were sunk in the Atlantic, and 15&nbsp;U-boats were destroyed. By May, wolf packs no longer had the advantage and that month became known as [[Black May (1943)|Black May]] in the [[U-boat Arm]] ({{lang|de|U-Bootwaffe}}). The turning point was the battle centred on slow [[convoy ONS 5]] (April–May 1943). Made up of 43&nbsp;merchantmen escorted by 16 warships, it was attacked by a pack of 30&nbsp;U-boats. Although 13&nbsp;merchant ships were lost, six U-boats were sunk by the escorts or Allied aircraft. Despite a storm which scattered the convoy, the merchantmen reached the protection of land-based air cover, causing Dönitz to call off the attack. Two weeks later, [[Convoy SC 130|SC 130]] saw at least three U-boats destroyed and at least one U-boat damaged for no losses. Faced with disaster, Dönitz called off operations in the North Atlantic, saying, "We had lost the Battle of the Atlantic".{{sfn|Costello|Hughes|1977|p=281}}
[[File:Vickers Wellington Leigh Light.jpg|thumb|A Vickers Wellington equipped with an ASV III radar under the chin and a Leigh light under the belly]]
On 13 April [[RAF Coastal Command]] started its second Bay Offensive with operation Derange. Seventy-five long range aircraft equipped with the new centrimetric [[ASV Mark III radar]] with [[Plan position indicator|PPI]] display patrolled regions in the [[Bay of Biscay]] with known concentrations (through enigma decrypts) of U-boats in transit. The German metox radar detector operated only in the metric band and did not detect the new centrimetric radar emissions. As a result, many U-boats were surprised and attacked. In response Dönitz ordered his U-boats to stay on the surface and fight it out with the aircraft. Some U-boats were converted to [[German Type VII submarine#U-flak "Flak Traps"|"''flak'' boats"]] with extra and new anti-aircraft guns, but to no avail: In May five U-boats were sunk and another seven were forced to abort.{{sfn|Costello|Hughes|1977|pp=287}}{{Sfn|Blair|1996b|pp=318-321}}

In all, 43&nbsp;U-boats were destroyed in May, 34 in the Atlantic. This was 25% of German U-boat Arm's total operational strength. The Allies lost 58&nbsp;ships in the same period, 34 of these (totalling 134,000&nbsp;tons) in the Atlantic.

===Convergence of technologies===
{{See also|RAF Coastal Command during World War II}}The Battle of the Atlantic was won by the Allies in two months. There was no single reason for this; what had changed was a sudden convergence of technologies, combined with an increase in Allied resources.

The [[mid-Atlantic gap]] that had previously been unreachable by aircraft was closed by long-range B-24 Liberators. On 18 March 1943, Roosevelt ordered King to transfer 60 Liberators from the Pacific theatre to the Atlantic to combat German U-boats; one of only two direct orders he gave to his military commanders in WWII (the other was regarding [[Operation Torch]]).{{sfn|Smith|2012|p=213}} At the May 1943 Trident conference, Admiral King requested General [[Henry H. Arnold]] to send a squadron of ASW-configured B-24s to Newfoundland to strengthen the air escort of North Atlantic convoys. General Arnold ordered his squadron commander to engage only in "offensive" search and attack missions and not in the escort of convoys. In June, General Arnold suggested the Navy assume responsibility for ASW operations. Admiral King requested the Army's ASW-configured B-24s in exchange for an equal number of unmodified Navy B-24s. Agreement was reached in July and the exchange was completed in September 1943.{{sfn|Bowling|1969|p=52}}[[File:U507_under_attack_1943.jpg|thumb|left| [[German submarine U-507|''U-507'']], under attack by a US Navy Consolidated PBY-5A Catalina of Patrol Squadron VP-83 off the northern coast of Brazil in the South Atlantic.]]

Further air cover was provided by the introduction of [[merchant aircraft carrier]]s (MAC ships), which carried [[Fairey Swordfish]], and although these Swordfish did not manage to score a U-boat kill, their presence deterred or drove off U-boat attacks as well as raising the morale of the merchant marine. Soon there were growing numbers of American-built escort carriers, primarily flying [[Grumman F4F Wildcat]]s and [[Grumman TBF Avenger]]s. Both types of carriers sailed with the convoys and provided much-needed air cover and patrols all the way across the Atlantic, plus escort carriers often formed hunter-killer groups not tied to a convoy getting them the freedom to seek out U-boats.<ref name=terraine>{{cite book
 | last = Terraine
 | first = John
 | title = Business in Great Waters: The U-Boat Wars 1916–1945
 | publisher = Mandarin Paperbacks
 | year = 1990
 | location = London, UK
 | pages = 756
 | isbn = 0-330-26172-X }}</ref>

Larger numbers of escorts became available, both as a result of American building programmes and the release of escorts committed to the North African landings during November and December 1942. In particular, [[destroyer escort]]s (DEs) (similar British ships were known as ''[[frigate]]s'') were designed to be built economically, compared to [[Destroyer#World War II|fleet destroyers]] and sloops whose warship-standards construction and sophisticated armaments made them too expensive for mass production. Destroyer escorts and frigates were also better designed for mid-ocean anti-submarine warfare than corvettes, which, although maneuverable and seaworthy, were too short, slow, and inadequately armed to match the DEs. Not only would there be sufficient numbers of escorts to securely protect convoys, they could also form hunter-killer groups (often centred on escort carriers) to aggressively hunt U-boats.

During May 1943, the US Navy began using a high-speed bombe of its own design which could deduce the settings of the new four-rotor German Enigma cipher machines.  By September 1944, 121 of the new high-speed bombes were at work.<ref name=Wenger1945 />

Dönitz's aim in this tonnage war was to sink Allied ships faster than they could be replaced; as losses fell and production rose, particularly in the United States, this became impossible.