The Battle of the Atlantic, which lasted from September 1939 until the defeat of Germany in 1945, was the war's longest continuous military campaign. During six years of naval warfare, German U-boats and warships – and later Italian submarines – were pitted against Allied convoys transporting military equipment and supplies across the Atlantic to Great Britain and the Soviet Union. This battle to control the Atlantic shipping lanes involved thousands of ships and stretched across thousands of perilous square miles of ocean.
Early in the war German warships made a number of forays into the shipping lanes, aiming to catch and destroy Allied convoys. These had limited success, and led to the loss of major ships including Graf Spee and Bismarck. From 1940 onwards, the German navy focused on escalating the U-boat war. Attacking on the surface at night (where they could not be detected by Allied sonar, or ASDIC), U-boats had great success against Allied convoys, sinking merchant ships with torpedoes and then submerging to evade the counterattack by escorting warships. They were also benefitting from decoded communications of the British Admiralty. In 1941 they inflicted huge losses, sinking 875 Allied ships.
During 1941, tactical advantage began to shift towards the British. They had received 50 American destroyers in exchange for US access to British bases. Canadians increased their escort missions, and RAF Coastal Command was able to increase its air cover. The capture of U-110 (complete with Enigma machine and codes) in March 1941 helped the Allies track the movement of German U-boats. From April 1941, US warships began escorting Allied convoys as far as Iceland, sparking a number of skirmishes with U-boats. This provoked controversy as the US had not officially entered the war. Technological developments, including radar for escorting warship from August 1941 (which could detect a U-boat periscope at a range of one mile) also worked in the Allies' favour.
Yet convoys were still very vulnerable in the 'Atlantic Gap', a 'black pit' in the mid-Atlantic which was not covered by anti-submarine aircraft. The gradual improvement of antisubmarine techniques, and the increased use of improvised aircraft carriers like HMS Audacity, led to a marked decrease in sinkings towards the end of the year. This contributed to Hitler's decision – carried out against Dönitz's wishes - to transfer submarines to the Mediterranean.
In 1942 the balance tilted once again in favour of the Germans. New submarines were entering service quickly, at a rate of 20 per month. Although the US Navy entered the war at the end of 1941, it was unable to prevent the sinking of almost 500 ships between January and June 1942. Allied losses in the Atlantic reached their peak in 1942. As 1,664 ships were sunk, supplies of petrol and food to Britain reached critically low levels.
In 1943, advantage shifted to the Allies once again. By now, the Allies had sufficient escort aircraft carriers and long-range aircraft to cover the Atlantic Gap. The battle reached its peak between February and May 1943. The 'hedgehog' depth-charge mortar was just one innovation that was making life more and more dangerous for U-boat crews. By 'Black May' of 1943, U-boat losses were unsustainable – one quarter of their strength in one month, and almost at the same rate as Allied shipping. U-boats were withdrawn from the Atlantic, and the battle was won. Although new German submarines arrived in 1945, they came far too late to affect the course of the battle.
Historians estimate that more than 100 convoy battles took place during the war. They cost the Merchant Navy more than 30,000 men, and around 3,000 ships. The equally terrible cost for the Germans was 783 U-boats, and 28,000 sailors.