After the fall of France in June 1940, one stark fact was clear. Without the ability to receive large quantities of material from North America, Britain was doomed. This meant, of course, that the subsequent ‘Battle of the Atlantic’ was one of the most decisive battles of the entire conflict.
‘Churchill himself said at the time,’ says Professor David Reynolds, ‘and certainly says in his memoirs, that he wasn’t really afraid of invasion in 1940. He could imagine the possibility of big raids or something like that on the east coast, but a full scale invasion was less worrying to him than the biggest fear that he had all through the war, which was that Britain’s sea lanes would be cut off, and that the country would eventually be starved into submission.
‘The British had assumed that there would be a French army holding the continent and that Britain’s role would be in support of that through naval and air power and so on. Suddenly the French just simply disappeared from the equation, Hitler is in control of continental Europe and if Britain wants to carry on the war it’s going to need at the very least large scale economic aid from the United States and probably military support as well, an army and all the rest of it.’
But, of course, the German Kriegsmarine intended to do all they could to prevent supplies reaching Britain across the Atlantic. And the capture of French territory made their task a good deal easier. The Germans were able to establish naval bases at several locations on the west coast of France, like La Rochelle and St Nazaire. From there, German submarines – U-boats – could be in the hunting grounds of the North Atlantic within hours.
And it was German U-boats that were to pose by far the biggest threat to Allied shipping during the war. Not only had the German surface fleet been badly mauled in Norwegian waters in early 1940, but the vulnerability of large warships like the Bismarck – which was sunk in May 1941 – both to air and surface attack, meant that the era of the great marauding battleships was over.
But still, it is something of a myth that the German U-boats were, in fact, predominantly underwater vessels at all. ‘The fact is that the submarines in this last war, they were actually not what you would call submarines nowadays,’ says Juergen Oesten, one of the most famous German U-boat commanders of the war. ‘They were surface vessels which could dive. But the problem was that when they went under the surface they were practically stationery, as they could manage only very slow speeds when under the surface. So the normal procedure, all over the globe wherever submarines tried to achieve something, was we tried to keep contact with the target during daytime and attacked eventually at night time on the surface. And this gave us a good chance in the first years of the war. With these three [different] boats I had during the war I sank 20 ships. But 19 I sank at night on the surface.’
With hindsight, it’s clear that the best chance the Germans had of severing the Atlantic link was during the nine months from June 1940 onwards. America was not in the war and Roosevelt had not yet firmly committed substantial American resources to help the British merchant marine effort. But Karl Doenitz, commander of the U-boat service, had less than 30 submarines at his disposal in the summer of 1940 and less than half of these were actually in the Atlantic waging war on the British at any one time.
But these small numbers of U-boats did have successes, particularly in the substantial ‘air gap’ that existed in the centre of the Atlantic where the convoys were beyond the range of aircraft patrols. Here the U-boats would form ‘wolf packs’ in preparation for an attack.
Attacking, for the most part, on the surface at night, the U-boats could devastate a convoy. In October 1940, for example, in convoy SC7 bound for Liverpool, 20 ships were sunk out of a total of 35. But, ultimately, there were simply not enough U-boats in the Atlantic at this stage in the war to cause Britain to collapse through lack of supplies. And by the time U-boat numbers did start to grow, during 1941, President Roosevelt had decided to support the British lifeline with substantial numbers of American escort vessels and merchant ships. This caused such a problem for the German navy – since America was still supposedly neutral how did they now know which ship was safe to attack – that Admiral Raeder, overall commander of the Kriegsmarine, was one of the few Germans who was overtly pleased when Hitler declared war on the United States in December 1941. Now, every ship the Germans saw in an Allied convoy in the Atlantic was fair game.
In the weeks after the Americans formally entered the war the U-boats enjoyed great success, attacking relatively unprepared vessels off the eastern coast of the United States. As a result, in 1942 the Germans sank more than eight million tons of shipping. But despite one more period of success – briefly in the spring of 1943 – the era of the U-boats was over. This wasn’t just because the mighty resources of American shipyards were now building merchant ships as fast – or faster – than the Germans could sink them, but because the Allies had changed their tactics. A combination of increased air support, military intellligence, radar, and wireless detection proved the death of the German submarine effort in the Atlantic.
In particular, the breaking of the German Naval Enigma code would play an important part in the reduction of the U Boat threat. Code-breakers at Bletchley Park had managed to read German U-boat messages for the first time in the summer of 1941, but subsequently there were periods when the intelligence was lost after the Germans had changed the code. It was in just such an intelligence gap, in March 1943, that the U-boats managed their last major success, attacking around half of the convoys then in the Atlantic. But it was a false dawn. The Allies re-broke the Enigma code, and a combination of factors – radar and wireless detection finding equipment on board Allied escort ships – plus increased Allied air support for convoys, meant that Karl Doenitz had little choice but to order his submarines out of the Atlantic.