Battle of the Atlantic (1939–1945): U-Boats, Convoys and Codes

The Battle of the Atlantic was the long struggle between Germany and the Allies for control of the Atlantic Ocean during the Second World War, from September 1939 to May 1945. German U-boats tried to sink merchant ships faster than they could be built, in order to cut Britain off from food, fuel and weapons from North America. The Allied victory, decided mainly in 1943, kept Britain in the war and made the invasion of Western Europe possible.

Background: an island that depended on the sea

Britain imported most of its oil, much of its food and many raw materials by sea. In the First World War German submarines had come close to starving the country, as the sinking of the Lusitania and the crisis of 1917 showed. The answer then had been the convoy system, in which merchant ships sailed in groups protected by warships.

In 1939 Germany’s navy was small, but Karl Dönitz, commander of the U-boat arm, argued that a large fleet of submarines could win a war against Britain. On the first evening of the war, 3 September 1939, U-30 torpedoed the liner Athenia without warning, killing more than 110 people. Britain introduced convoys at once. In October Günther Prien’s U-47 sank the battleship Royal Oak at Scapa Flow, and German surface raiders were also hunted down, as in the Battle of the River Plate.

The “Happy Times” and the wolfpacks

The fall of France in June 1940 transformed the campaign. U-boats could now operate from bases on the French Atlantic coast such as Lorient, Brest and Saint-Nazaire, far closer to the convoy routes. Britain, short of escorts after Dunkirk, suffered heavy losses in what U-boat crews called the first “Happy Time”. Commanders such as Otto Kretschmer became famous in Germany.

Dönitz developed wolfpack tactics. U-boats formed patrol lines across the expected routes; when one sighted a convoy, it reported by radio, and headquarters directed others to attack together at night on the surface, where they were hard to detect with sonar. Churchill named the struggle “the Battle of the Atlantic” in a directive of March 1941. He later wrote that “the only thing that ever really frightened me during the war was the U-boat peril”.

The Royal Navy and the rapidly growing Royal Canadian Navy carried most of the escort burden. In September 1940 the United States transferred 50 old destroyers to Britain in exchange for base rights, and the Lend-Lease Act of 1941 increased the flow of American supplies. After Germany declared war on the United States in December 1941, U-boats struck the poorly defended American east coast, sinking hundreds of ships in a “second Happy Time” until coastal convoys were organised later in 1942.

Ships, aircraft and codes

The campaign was a contest of technology and intelligence as much as of courage. Escorts used ASDIC (sonar) to detect submerged submarines and depth charges, and later the ahead-throwing Hedgehog, to attack them. High-frequency direction finding, known as “Huff-Duff”, located U-boats when they transmitted. Compact centimetric radar, made possible by the cavity magnetron, could detect a surfaced U-boat at night, and Leigh Lights on aircraft illuminated them in the dark.

Codebreakers at Bletchley Park read naval Enigma for much of 1941, helped by material captured from U-110 and German weather ships. In February 1942 a new four-rotor Enigma network for U-boats, known to the British as Shark, blocked them for most of the year until captured documents from U-559 helped restore access in December. Germany’s own naval intelligence service, the B-Dienst, meanwhile read the British convoy code until it was changed in June 1943.

The turning point of 1943

Losses peaked in 1942, when U-boats sank over six million tons of shipping. Much damage was done in the mid-Atlantic “air gap”, beyond the reach of land-based aircraft. In March 1943 the battles around convoys HX 229 and SC 122 seemed to show the U-boats winning.

Within weeks the balance changed. Very long-range Liberator aircraft and small escort carriers closed the air gap, and support groups of escorts under Admiral Max Horton’s Western Approaches Command hunted U-boats aggressively. Around convoy ONS 5 in late April and early May 1943 several U-boats were sunk for the loss of 13 merchant ships. In May, remembered in Germany as Black May, around 40 U-boats were lost, and on 24 May Dönitz withdrew from the North Atlantic.

U-boats fought on until the end, with snorkels and new designs, but never regained the initiative. American shipyards, building standard Liberty ships at enormous speed, were by then producing far more tonnage than Germany could sink.

Key people

• Karl Dönitz commanded the U-boat arm throughout the war and became head of the whole German navy in January 1943. He briefly succeeded Hitler as head of state in May 1945.
• Winston Churchill, as First Lord of the Admiralty and then Prime Minister, followed the shipping figures closely and gave the campaign high priority.
• Admiral Sir Percy Noble and, from November 1942, Admiral Sir Max Horton, a former submariner, led Western Approaches Command from Liverpool, which directed the convoy battles.
• Captain Frederic John Walker became Britain’s most successful U-boat hunter, leading escort groups that sank around 20 submarines.
• Günther Prien and Otto Kretschmer were among the best-known U-boat commanders; Prien was lost with U-47 in March 1941, and Kretschmer was captured the same month.

Consequences

Victory in the Atlantic allowed millions of American and Canadian soldiers and vast quantities of equipment to cross to Britain for the D-Day landings. It also kept supplies flowing on the dangerous Arctic route to the Soviet Union. The cost was very high on both sides, especially for merchant seamen, who served in unarmed or lightly armed ships.

U-boat crews suffered one of the highest loss rates of any service in the war. From 1943 onwards a sailor joining a U-boat had a poor chance of surviving, as Allied aircraft and escort groups caught submarines on passage across the Bay of Biscay and around the convoys.

Historians still debate how close Germany came to winning. Britain never ran out of food or fuel, and Allied shipbuilding eventually far outstripped losses. Many scholars now argue that the U-boats came closest to success not by starving Britain but by delaying the build-up of Allied forces for the invasion of Europe.

By the numbers

Timeline

• 1939, 3 September: U-30 sinks the liner Athenia on the first day of the war.
• 1940, June: The fall of France gives U-boats Atlantic bases.
• 1941, 6 March: Churchill issues his Battle of the Atlantic directive.
• 1941, 9 May: Enigma material is captured from U-110.
• 1942, January: U-boats begin attacks off the American east coast.
• 1943, March: The battles for convoys HX 229 and SC 122 bring heavy Allied losses.
• 1943, 24 May: Dönitz withdraws U-boats from the North Atlantic after Black May.
• 1945, 8 May: Germany surrenders and U-boats are ordered to surface and give up.

Why it matters

The Battle of the Atlantic was the foundation of the Western Allied war effort: without the sea lanes, Britain could not have fought on and the liberation of Western Europe could not have been mounted. It also showed how science, industry and intelligence could decide a military campaign, a lesson that shaped navies and anti-submarine warfare long after 1945.