'WITHOUT VICTORY, THERE IS NO SURVIVAL'

BITTERLY fought over three million square miles of hostile ocean, the struggle to prevent Hitler's U-boats - his 'grey wolves' - starving Britain into submission began on the first day of the war and ended on the last.

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Casting his mind back over five bloody and uncertain years, Winston Churchill later declared the "U-boat peril" was the only thing that ever really frightened him. Battles might be won or lost, but the country's very existence depended on the Atlantic. As the PM said in June 1940: "Without victory there is no survival."

Yet despite the existential threat, little was reported at the time. In the words of one Royal Navy veteran, it was an "unseen war", and the seamen who paid the ultimate price have no grave but the ocean.

When war began, the country looked to the Royal Navy, historically the world's most powerful, for protection.

The navy was confident it could prevent vital imports reaching Germany and, at the same time secure, Britain's lifeline. Before the war, we imported 60 million tons of food and raw materials a year and every drop of oil - most across the Atlantic from America via a merchant fleet of 3,000 vessels.

By concentrating ships into fleets of 30 or more, the convoy system was able to reduce the number of targets for the enemy. What's more, the Admiralty was confident its new echo-sounding sonar device would strip German subs of their “cloak of invisibility”.

The U-boat had taken Britain to the brink of defeat during the First World War, but by 1939 the German navy was a shadow of its former self with only 27 subs capable of Atlantic operations. Their commander, Karl Dönitz, was confident a larger fleet could win the war, but he needed “the boldest of bold enterprises” to convince Hitler.

His plan was an attack on British battleships in their historic home base, Scapa Flow in Orkney.