The Battle of the Atlantic began within hours of Britain declaring war on Germany on September 3rd 1939 when the German U-Boat U-30 fired on the SS Athenia, a British liner carrying passengers from Liverpool to Montreal causing the loss of 112 lives.
What followed was a war over supplies. Britain needed over a million tons of food, oil and raw materials for her industries to arrive every week. With European ports under German control, all of Britain's vital supplies would now have to come across the Atlantic from Canada and the USA. German Naval Command calculated that if 750,000 tons of British shipping could be sunk each month, Britain would have to surrender within a year.
In October German planes begin dropping magnetic mines from the air, mines triggered by the weak electric field of a ship's hull. These proved devastatingly effective but the lucky find of an unexploded mine on a beach near Southend allowed British scientists to develop a defence against them within weeks.
British losses continued to mount. British ships were at risk not only from attack by U-boats, hidden beneath the water, but from German surface ships. The pocket battleship the Graf Spee alone sank nine ships in the last three months of 1939 before being heavily damaged and sunk off the coast of Uruguay.
The most dangerous time for British convoys was at the centre of the Atlantic, in what became know as the mid-Atlantic gap. This gap fell between the furthest ranges of allied aircraft operating from Newfoundland and the coast of Canada and from Britain. Inside it, Allied shipping was especially vulnerable.
In May 1940 Britain occupied Iceland and increased the range of its air support but the gap remained with British losses running at 300 ships a month. In contrast only around 20 U-Boats were sunk in the whole of 1940.
At the orders of their Commander, Admiral Karl Donitz, U-Boats began to hunt down enemy shipping in packs making them effective against even large, well protected convoys.
Only a few facts offered hope to the British. The first was the continued success of the Royal Navy against German surface ships. On 27th May 1941, in return for her destruction of HMS Hood, a force of 16 British ships hunted down the German battleship Bismarck and sank her. Bismarck's destruction caused gloom in Berlin and was a massive British propaganda victory.
The second was the growing success of the top secret Station X at Bletchley Park in Buckinghamshire in cracking the codes used by German ships and the German high command to send messages. This ability to read the orders given to German ships began to allow convoys to avoid attack and Royal Navy destroyers to find targets more easily.
In 1941 the United States began to become more directly involved in the war at sea. American ships were sent to protect convoys at sea as far as Iceland, where they would be met by British vessels. In October this led to the first sinking of an American vessel by a U-Boat when U-552 torpedoed the USS Reuben James as it moved to protect a convoy carrying ammunition.