Battles & Wars › World War II › World War II
Battle of the Atlantic
September 3, 1939 – May 8, 1945· Battle· Europe
Well documentedSolid records, physical evidence or scientific consensus.
World War II's longest continuous campaign: German U-boats tried to cut the sea lanes that fed Britain and supplied Allied armies.
See it on the Battles & Wars map →What happened
Britain imported most of its oil and about half its food. German U-boats, attacking convoys in 'wolfpacks', often on the surface at night, sank about 3,500 Allied merchant ships. Losses peaked in 1942 and early 1943. Then escort carriers, long-range aircraft closing the mid-Atlantic 'air gap', centimetric radar, broken Enigma codes and better escort tactics turned the tide. In 'Black May' 1943 about 40 U-boats were lost, and Admiral Karl Dönitz pulled them out of the North Atlantic.
Why it matters
Had it been lost, Britain could not have fought on and no American army could have crossed to invade Europe. Churchill said the U-boat peril was the only thing that really frightened him.
How we know
Admiralty and German naval records, convoy logs and wrecks on the seabed. Estimates vary: roughly 72,000 Allied naval and merchant sailors died, and about 28,000–30,000 of some 40,000 U-boat crewmen.
Still debated
How much credit goes to codebreaking versus radar, aircraft and sheer shipbuilding is still argued.
Connections (9)
▸Battle of the Atlantic was fought against U-boats and unrestricted submarine warfareWell documented
The connection
Germany revived the unrestricted submarine warfare of World War I, sinking merchant ships without warning. Admiral Karl Dönitz grouped U-boats into wolfpacks that shadowed convoys and attacked together at night. The Allies answered with the remedy that had worked in 1917, convoys, now backed by radar, aircraft and codebreaking.
When and how
North Atlantic, Arctic and American coastal waters, 1939–1945; Dönitz's U-boat command against British, Canadian and US navies and air forces.
How we know
German U-boat command war diaries, Allied convoy records, and the Nuremberg trial, where Dönitz's submarine orders were examined.
Debates and open questions
Some argue that if Germany had built many more U-boats before 1939, as Dönitz wanted, Britain could have been starved out.
▸Battle of the Atlantic kept Britain supplied throughout World War IIWell documented
The connection
Britain's war effort depended on oil, food, weapons and troops shipped from North America and the empire. Keeping the Atlantic lanes open was the precondition for everything Britain, and later the United States, did in Europe, from the desert war in North Africa to the bombing of Germany and the invasion of Normandy.
When and how
The whole Atlantic, from September 3, 1939, when U-30 sank the liner Athenia, to Germany's surrender in May 1945.
How we know
British import and rationing records show how close supplies came to failing in late 1942 and early 1943.
Debates and open questions
Historians debate how near Britain really came to starvation; some argue the danger was overstated because imports never fell to the critical level.
▸Battle of the Atlantic had to be won before D-Day (Normandy landings)Mostly agreed
The connection
An invasion of France needed about 1.5 million American troops and millions of tons of supplies shipped to Britain first. That was only possible once the U-boats were beaten in mid-1943. By June 1944 convoys crossed with few losses, and almost no U-boats got through to the Normandy invasion fleet.
When and how
North Atlantic convoy routes, mid-1943 to June 1944; Operation Bolero, the American buildup in Britain.
How sure are we?
Allied planning papers made winning the shipping war a condition for invasion, and US troop numbers in Britain rose sharply once losses fell.
Debates and open questions
Whether D-Day could have come in 1943, as Stalin and some US planners wanted, depends partly on how early the Atlantic was really secure.
▸Battle of the Atlantic pitted U-boats against escorts using Sonar (ASDIC)Well documented
The connection
Britain entered the war trusting ASDIC, but U-boats often attacked on the surface at night, where sonar could not find them, and contact was lost as an escort ran over its target to drop depth charges. Forward-firing Hedgehog mortars, radar, aircraft and broken codes filled the gaps, and the U-boats were decisively beaten in May 1943.
When and how
North Atlantic convoy routes, 1939–45; British, Canadian and US escorts, such as Captain Frederic 'Johnnie' Walker's group, against Admiral Karl Dönitz's U-boats.
How we know
Naval action reports, U-boat logs and postwar studies matching each sinking to its cause, which show sonar played a part in many kills.
Debates and open questions
How to divide the credit among sonar, radar, codebreaking, aircraft and sheer numbers of escorts is still debated.
▸Bletchley Park and Ultra codebreaking read U-boat orders during the Battle of the AtlanticWell documented
The connection
U-boats were steered from shore by radio, so reading their Enigma traffic let the Admiralty route convoys around wolfpacks. In February 1942 the Germans added a fourth rotor to naval Enigma, and Bletchley went largely blind for about ten months while losses soared. Breaking the new 'Shark' key in December 1942 helped turn the tide by May 1943.
When and how
Hut 8 at Bletchley Park and the Admiralty's Operational Intelligence Centre in London, 1941–45; codebooks seized from U-110 (May 1941) and U-559 (October 1942).
How we know
Declassified Bletchley and Admiralty records show the blackout and the drop in convoy losses when U-boat signals were being read.
Debates and open questions
How much of the 1943 victory came from Ultra versus radar, aircraft and escort tactics is still weighed by historians.
▸Cavity magnetron and the Tizard Mission let patrol aircraft find U-boats in the Battle of the AtlanticWell documented
The connection
Earlier long-wave airborne radar gave weak returns and could be detected by U-boat receivers. Centimetric radar based on the magnetron, such as Britain's ASV Mark III from early 1943, could spot a surfaced submarine at night, and Germany had no warning receiver for it for months. Paired with the Leigh searchlight, aircraft caught U-boats surfacing to recharge batteries.
When and how
Bay of Biscay and North Atlantic, 1943; RAF Coastal Command and US aircraft, plus escort ships fitted with Type 271 centimetric radar from 1941.
How we know
Admiralty and Coastal Command records, and German U-boat command diaries complaining of surprise night air attacks.
Debates and open questions
Dönitz blamed radar for losses partly caused by codebreaking, which German inquiries repeatedly ruled out; separating the two causes remains hard.
▸Enigma machine encrypted the U-boat orders of the Battle of the AtlanticWell documented
The connection
U-boats reported positions and received orders from Dönitz's headquarters by radio, encrypted on naval Enigma, which was harder to break than army versions. Wolfpack tactics depended on this constant radio traffic. That made the U-boat fleet both effective and vulnerable: whenever Bletchley could read it, convoys could be routed away.
When and how
U-boat headquarters in France and Germany and boats at sea, 1939–45; the three-rotor naval Enigma and, from February 1942, the four-rotor M4.
How we know
German naval radio logs and captured machines and codebooks, notably from U-110 and U-559, confirm how the system worked.
Debates and open questions
Why German naval intelligence, which had its own suspicions, never concluded that Enigma was broken is still discussed.
▸Liberty ships outpaced U-boat sinkings in the Battle of the AtlanticWell documented
The connection
In 1942 U-boats sank Allied ships faster than they could be replaced. US yards then raised output so quickly that, during 1943, new ship tonnage overtook losses for good. Liberty ships made up most of the new cargo tonnage; about 200 of them were lost to enemy action during the war.
When and how
US shipyards and the North Atlantic, 1942–43; Henry Kaiser's yards in Richmond, California, and Portland, Oregon, among 18 yards.
How we know
Maritime Commission tonnage figures and Allied shipping-loss tables show the crossover point.
Debates and open questions
Historians weigh how much the Atlantic was won by building ships faster versus sinking U-boats; both were needed.
▸MIT Radiation Laboratory designed microwave radars that hunted U-boats in the Battle of the AtlanticWell documented
The connection
Rad Lab designs such as the SCR-517 and ASG airborne radars let US patrol planes find surfaced U-boats at night or in fog. Fitted to escort-carrier aircraft and long-range Liberators, they helped close the mid-Atlantic air gap in 1943. The lab also developed LORAN radio navigation, giving ships and aircraft accurate positions at sea.
When and how
Western and mid-Atlantic, 1942–43; US Navy and Army Air Forces anti-submarine squadrons.
How we know
Rad Lab histories and Navy records link specific radars to U-boat detections and sinkings.
Debates and open questions
British and American radars both claim credit, and with codebreaking and new tactics arriving in the same months, the radar share is hard to isolate.