The role of aeroplanes
Apart from ships, two other factors played a vital role in the winning of the Battle of the Atlantic. The first was the aeroplane. One of the major problems faced by the Allies in the early years of the war was the existence of a 'mid-Atlantic gap', an area that could not be reached by friendly aircraft.
It was crucial to find a way of reaching this area, as simply by flying over the sea, aeroplanes could force submarines to submerge and cease activity, and they could, of course, counter the Kondor. Early in the war, fighter aircraft such as the 'Hurricane' could be carried to the mid-Atlantic, and catapulted from the decks of specially adapted ships (known as Catapult Aircraft Merchant ships, or CAMs), although these were 'one-shot weapons', the planes having to ditch in the sea afterwards. Light escort carriers, also capable of carrying aircraft, entered service in September 1941, and these were a major step forward.
The role of long-range aircraft such as the American 'Catalina' flying boat was also crucial for the battle in the mid-Atlantic area, although there is a big question mark over whether the Allies made the best use of their available aircraft.
The B-24D 'Liberator', a very-long-range aircraft, was the victim of a power struggle within the RAF. At first it was used only for the strategic bombing of Germany, the dominant strategy within the RAF at that time, and was only released to Coastal Command towards the end of 1942.
The argument of 'Bomber' Harris had been that the RAF's most useful contribution to the Battle of the Atlantic was to bomb the U-boat pens and production facilities on land - a view that was, and remains, deeply controversial. By the second half of 1943, however, as these longer-range aircraft were released for the sea battle, the mid-Atlantic gap was at last being satisfactorily covered.
The crisis of the Battle of the Atlantic came in early 1943. Döntiz, by this time commander of the German Navy, now had 200 operational U-boats. British supplies, especially of oil, were running out, and it became a question of whether Allied shipyards could build merchant ships fast enough to replace the tonnage that was being sunk. Mass production of Liberty Ships in US shipyards, however, helped to ensure that the Allies would win this race.
At sea, the situation was saved by aggressive anti-submarine tactics, by new technology - better weapons and radio, the long-range aircraft Liberator being equipped with centimetric radar - and, eventually, by a revived Ultra intelligence.
By April the U-boats were clearly struggling to make an impact. Even worse, from Hitler's point of view, was the fact that Allied sinkings of German submarines began to escalate, with 45 being destroyed in the months of April and May. Döntiz, recognising that the U-boat's moment had passed, called off the battle on 23 May 1943.
This was not the end of the threat in the Atlantic, but thereafter it was greatly diminished. After his withdrawal, Hitler insisted on keeping troops on the Baltic coastline, even after they had been cut off by advancing Soviet troops, in order to maintain possession of a testing ground for new types of U-boats. By the end of the war, the Germans had indeed produced new types of 'super-submarines', the Types XXI and XXIII, and these would have been very dangerous had they been introduced earlier.
The Battle of the Atlantic was one of the longest campaigns of World War Two, and it was proportionally among the most costly. Between 75,000 and 85,000 Allied seamen were killed.
About 28,000 - out of 41,000 - U-boat crew were killed during World War Two, and some two-thirds of these died in the course of the Battle of the Atlantic. The stakes could not have been higher. If the U-boats had prevailed, the western Allies could not have been successful in the war against Germany.