The conflict between Allied ships and German U-boats was dubbed the Battle of the Atlantic by Winston Churchill on 6 March 1941.

In those grim days it was a brutal and merciless fight for Britain’s survival. Churchill said that the ‘U-boat peril’ was the only thing that ever really frightened him during the Second World War. Official figures underline why – more than 2,500 ships were sunk by U-boat action during the Battle of the Atlantic. Britain’s low point came in 1942 when shipping losses were running at 650,000 tons per month – dangerously close to the 800,000 ton target that the Germans believed would starve Britain to submission.

More than any other weapon at Hitler’s disposal – including the panzer divisions of the Blitzkrieg and the heavy bombers Luftwaffe – the U-boat came closest to defeating Britain. It was a remarkably unsophisticated vessel, not far removed from the early U-boats of the First World War. And yet it proved a lethal and elusive adversary, capable of inflicting immense destruction – especially when Admiral Donitz’s wolfpacks closed in on slow-moving and poorly protected convoys.

Nevertheless, the U-boats did not have it all their own way. The U-boat arm deployed 860 operational vessels, of which 750 were lost – an appalling attrition rate of 87 per cent. And when a U-boat went down, few survived. Some 39,000 men served in the U-bootwaffe, of whom nearly 29,000 died in action – that’s three out of four of all German submariners.

This vast and pitiless conflict is described at first-hand by Royal Navy Coder Cliff Greenwood in High Seas to Home. And it is also part of the setting for my upcoming spy thriller Emerald Reich.

An authentic insight into the tactics used on both sides is the 1957 movie The Enemy Below, starring Robert Mitchum as the skipper of an American escort and Curt Jurgens as the U-boat commander he must defeat.The reason why The Enemy Below is especially illuminating is that it was the first movie that portrayed a U-boat crew in a sympathetic light – as brave and resourceful individuals fighting honourably, rather than the familiar stereotype of ruthless Nazi fanatics. As a result, you get a compelling insight into what it must have been like for the combatants – both above and below the waves.

For a purely U-boatman’s perspective, one need look no further than the peerless Das Boot. Whether you’re reading Lothar-Gunther Buchheim’s 1973 novel or watching Wolfgang Petersen’s multi-Oscar nominated 1982 movie, the experience is unforgettable. You can almost smell the diesel engines, the filthy bilge water, the reek of unwashed men toiling in their ‘steel coffin’. Conditions were unbearable – there were only two toilet cubicles for the whole crew, and one of these was out-of-use because it was crammed with supplies. Food was fine – until the fresh supplies ran out. As for alcohol, a U-boat crew of fifty could expect one bottle of Beck’s beer per man for the entire voyage.

When a U-boat went to action stations, the drill would seem chaotic to an outsider, but each member of the crew knew exactly what to do, and when. This often began with the order to drive which involved almost the whole crew rushing right forward into the torpedo room to maximise the angle of descent.

If an attack was successful, celebrations were usually short-lived because enemy warships retaliated swiftly and relentlessly. As the war progressed U-boat losses mounted. The ‘happy times’ of 1940-41 and the first half of 1942 were never coming back. The U-boat aces were mostly dead or taken prisoner and rookie crews put to sea in vessels with barely-dry paint.

Nonetheless, the U-boatmen fought hard and valiantly to the end, with Allied shipping losses rising markedly in the final months of the war.    

But the undoubted courage and daring of German submariners was never going to be enough. The U-boat campaign was ultimately defeated by an almost limitless supply of merchant ships from American yards, combined with increasingly effective Allied anti-submarine technology.