Battle to break the U-boat blockade: Lessons of history: A crucial phase of the Second World War hinged on military intelligence and technology, John A Terraine finds

THERE HAS been nothing quite like the Battle of the Atlantic and it is improbable that there ever will be. Captain Stephen Reskill, the Royal Navy's great Second World War historian, says categorically: 'In all the long history of sea warfare, there has been no parallel to this battle, whose field was thousands of square miles of ocean, and to which no limits in time or space could be set.' Sir Winston Churchill called it 'the dominating factor all through the war', and elsewhere he claimed that it was 'the only thing that ever really frightened me'.

The battle's true beginning was in June 1940, when the German army reached the French Atlantic coast as a result of the fall of France. The capture of the Bay of Biscay ports - Brest, Lorient, St Nazaire, La Pallice and Bordeaux - gave the German submarine fleet a starting-point towards the British trade routes in the Atlantic as far west as the tip of Cornwall. This saved 450 miles each way on the journey to the submarine cruising stations.

The German submarines, U-boats, were the prime enemy from beginning to end. Pocket battleships and other surface warships and aircraft played their parts at different times, but it was the U-boats that struck deep fear into Winston Churchill and the Admiralty. When they were present in strength, the issue of the battle - and sometimes of the war - was always in doubt; when they withdrew it was a victory. If they had not finally withdrawn, it would have been a disaster.

The U-boats performed their blockade of Britain (formally declared in August 1940) on the principle of Unrestricted Submarine Warfare enunciated in February 1917, and their favoured method - attack on the surface by night - was a well-tried First World War practice. The U-boats of 1918 and 1940 belonged to the same phase of the Industrial Revolution; they used the same sort of machinery, the same sort of weapons and much the same techniques.

Strictly speaking, neither of the U- boat wars was a submarine war. From 1914 to the end of 1945, U-boats were really submersibles - torpedo-boats, whose best performances of range and speed were on the surface, but possessing the invaluable ability to dive for attack or escape. The torpedo was the special weapon of the U-boats; it was the essence of Unrestricted Warfare. Attack by torpedo was stealthy, sudden and devastating, sending great ships to the bottom in a matter of minutes, sometimes with no survivors, sometimes followed by the slow death of crews and passengers by starvation or thirst in open boats and shelterless rafts, sometimes by burning in the blaze of oil-covered sea.

The guiding spirit of the Battle of the Atlantic was always Admiral Karl Donitz, commander of the U-boat fleet and later Commander-in-Chief of the German navy. He was heart and soul a submariner, devoted to that arm of the service, and he had been pondering and planning attacks on merchant convoys ever since the surrender of 1918. Donitz was convinced that 'a massed target should be engaged by massed U-boats', and from the very beginning of the war he looked for what he called 'one great success: for example, the destruction of a whole convoy'.

If he never achieved this, it was not for want of trying, but closely linked to the fact that he never wielded what he would consider to be 'massed U-boats'. On the contrary, his everlasting complaint was of what he called the 'dearth of U-boats'. Early in 1939, he had said that he would need 300 for an effective blockade of Britain; when war broke out he had 42 operational and for the rest of that year it was unusual for more than six or eight to be on station in the Atlantic at any given moment. In February 1941, his operational total dropped to a mere 22 U-boats. It then climbed slowly to a maximum of 239 in May 1943, the climax of the Atlantic battle - but by then Donitz knew that they were obsolete.

The first U-Boat pack attack on a convoy was upon SC2, homeward-bound from Sydney, Nova Scotia, in September 1940. The pack, although small (only four boats), contained two of the most famous U-boat 'aces': Gunther Prien (U47) and Otto Kretschmer (U99). The attack contained significant prophetic features, of which the most outstanding was its point of origin: a decrypt of an Admiralty signal to the convoy by xB-Dienst, the crypt-analysis section of German Naval Intelligence. Radio interception was essential to the 'Donitz system' of war; the information it supplied (known in Britain as 'Sigint') provided him with 'the only reconnaissance service on which I can rely'.

Radio itself gave him the method of command. Very early in the war he concluded that U-boat operations could be most effectively controlled by radio from the shore rather than by a flotilla-leader 'on the spot'. For this purpose the U-boat Command created a signals network which, according to the Official Intelligence History, 'for complexity, flexibility and efficiency, was probably unequalled in the history of military communications'. The weakness of the system was the enormous output of radio signals and the consequent security risks. Breaking that security was the function of Ultra, the Sigint decoded and translated at Bletchley Park and fed into the Admiralty's Operational Intelligence Centre to come to rest in the Submarine Tracking Room, under its brilliant chief, Commander Rodger Winn RNVR. Winn has been called the true opponent of Admiral Donitz in this very special war. I am not sure that modern warfare has room for such personal combats, but if one does accept a chess-board image, it is hard to see who else would be sitting on the British side of the board.

Having thus located the SC2 convoy, Donitz ordered U65 to trail the convoy and report its position. This brought in the rest of the pack, and the attack was launched before dawn on 7 September, on the surface. 'Before dawn' meant that the U-boats were practically invisible in the darkness of a wild night; 'on the surface' meant that the Royal Navy's Asdic (sonar) location system was rendered useless. This tactic took the Navy by surprise; it became standard drill throughout the coming battle.

All things considered, SC2 got off lightly: five ships lost out of 53. Others were less fortunate; on three successive days of October, three convoys (SC7, HX79 and HX79a) lost a total of 38 merchant ships. To appreciate the meaning of that figure we should recall the words of the Intelligence History: 'the severe mauling of a convoy was the equivalent of a lost battle on land'.

The battle continued on these lines. Its tactical essence was, for the U-boats, the problem of finding the convoys in the immensity of the ocean surface; for the convoy escorts, the problem of finding the U-boats in the vastness of the ocean depths. To both sets of problems, the answers could only be supplied by technology, and, for both, Sigint continued to be of central importance. In June 1941, the code-breakers at Bletchley Park cracked the Enigma ciphers, which carried the incessant radio traffic of the 'Donitz system'. The result, according to the German U-boat authority Professor Jurgen Rohwer, was that 'during the second half of 1941, by a very cautious estimate, the Submarine Tracking Room of the Admiralty . . . rerouted the convoys so cleverly around the German 'Wolf Packs' that about 300 ships were saved by avoiding battles. They seem to me more decisive to the outcome of the battle than the U-boats sunk in the convoy battles of 1943'.

It was in 1941 that the anti-U-boat headquarters moved to Derby House, Liverpool, where first Admiral Sir Percy Noble and then Admiral Sir Max Horton presided over an intimate joint Operations Room shared with the Air Officer commanding No 15 Group, Coastal Command.

The evident and continuing feature of the battle was now its steady move westward, further into the Atlantic, thanks to the increasing strength and lethal equipment of Coastal Command. Its Sunderlands, reinforced by Catalinas, Wellingtons and Whitleys, drove the U-boats to mid-ocean and a steadily narrowing area known as 'the Gap', the only place where U- boats could safely show their conning towers above the surface. Closing the Gap became an urgent priority and the struggle to obtain enough B24 (Liberator) aircraft was hard and bitter. Fortunately, it turned out that 'enough' was not very many.

Western Approaches Command made a deeply significant contribution to anti-submarine warfare in 1941. It defined at last, with impeccable clarity, what the object of the exercise was, thus concluding a long argument. This revolved around the question of whether convoy escorts were better employed trying to sink U-boats or trying to defend convoys - not always the same thing at all. In April, Western Approaches laid it down clearly: 'The safe and timely arrival of the convoy at its destination is the primary object and nothing releases the Escort Commander of his responsibility in this respect.'

'Safe and timely arrival' was not a keynote of 1942. Three times in that grimmest year of the war the monthly total of tonnage lost rose above 800,000, U-boats and now Japanese submarines being responsible for about three-quarters of it all.

Bletchley Park endured a Sigint blackout for most of the year when the Germans altered their Enigma settings. The awaited new weapons and the priceless additions of effective air- and sea-borne radar and High Frequency Direction-Finding took a long time to emerge from their pipelines. The number of operational U-boats was rising steadily. The crisis of the battle was at hand.

The month of March brought the rock bottom of Allied fortunes - a 20-day period during which 97 ships (over half a million tons) were lost, and a later Admiralty review admitted: 'The Germans never came so near disrupting communication between the New World and the Old as in those first 20 days of March 1943.'

And then the picture dramatically changed. Bletchley Park regained its ascendancy; the VLR (Very long Range) Liberators arrived - and also aircraft carriers in escort groups; weapons overcame their 'teething troubles'; Support Groups double- banked the convoy escorts. Thanks to the full Allied technological array and the courage, endurance and skill of the Royal Navy, the RAF and the merchant seamen, the U-boats began to lose more boats than they sank. Their morale broke suddenly, and on 24 May, when 31 of them had gone down, Donitz ordered them to withdraw. He admitted: 'We had lost the Battle of the Atlantic.'

The victory was obvious, but its full significance was not appreciated for a long time. It was widely regarded as a defensive victory in protection of Britain's lifelines. It was far more than that. From the moment when the American and British governments adopted their 'Germany First' strategies, its linchpin became an amphibious assault on north-west Europe, ultimately code-named Overlord. This would not have been possible with the U-boats still at large; the Atlantic victory must also be rated as the basic ingredient of the greatest western offensive of the war.

It was only just in time. By the end of 1944 the breakthrough to a new phase of technology had occurred; the Type XXI and Type XXIII U-boats threatened the anti-submarine forces with a clear view of Square One. The age of the submersible was over; the age of the submarine had begun.

John A Terraine is a military historian, whose books include Business in Great Waters: the U-boat Wars 1916-1945 (1989).

(Photograph omitted)