Battle of the Atlantic

• 1. Introduction

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  The Battle of the Atlantic was, by Winston Churchill's own admission, the thing that frightened him most during the entire Second World War. "I was even more anxious about this battle," he later wrote, "than I had been about the glorious air fight called the 'Battle of Britain'." That confession from a man who presided over the Blitz and oversaw the liberation of Europe tells you something essential about what was at stake in the North Atlantic. Britain required more than a million tons of imported material every single week just to survive and keep fighting. Cut that lifeline and the war was lost, not just for Britain but for the Allies. This was the premise on which Germany built its entire naval strategy, and the question that hung over the ocean from 1939 until the German surrender in May 1945: could a fleet of submarines strangle an island nation before the island nation's allies could stop them? The battle that followed has been called the "longest, largest, and most complex" naval battle in history. It spanned millions of square miles, pulled in thousands of ships, and ran for more than five years without pause. It was fought not just with guns and torpedoes but with codebreakers, mathematicians, radar engineers, and women playing war games on linoleum floors. By the end, 3,500 merchant ships and 175 warships lay on the ocean floor, alongside 783 U-boats and 47 German surface warships. To understand how the outcome swung back and forth, you have to start with a single man's calculations made before the shooting even began.

• 2. Dönitz and His Arithmetic

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  Karl Dönitz, commander of the German U-boats, had done the maths. He believed that 300 medium Type VII U-boats, sinking a million tons of shipping per month, could within a year destroy enough of Britain's roughly 3,000 merchant ships to collapse the British economy entirely. The number 17.5 million tons kept appearing in his calculations: that was the combined tonnage of those British merchant vessels, the aggregate that had to be ground down. Dönitz also had a tactical idea to match his strategic vision. In the First World War, U-boats had been beaten mainly by the convoy system, which concentrated escorts around merchant ships. Dönitz thought this could be defeated by what he called the Rudeltaktik: a patrol line of submarines spread across convoy routes, one boat finding the target, then radio-coordinating the rest to converge and attack together at night from the surface. ASDIC, the British sonar system, could not detect a surfaced submarine. Aircraft could not operate at night, and a U-boat could dive before a daylight attacker arrived. His plan was logically coherent. His superiors ignored most of it. Erich Raeder, chief of the German navy, kept choosing mixed submarine fleets instead of the concentrated force of medium boats Dönitz wanted. Even Hitler's ambitious Plan Z of 1938 allocated only a minority of its planned 239 submarines to the medium type Dönitz needed. When war came in September 1939, many of the 57 available U-boats were small short-range Type IIs suited mainly for minelaying in coastal waters. Dönitz had planned maximum effort for the opening month, almost all available boats out on patrol at once. That level of deployment could not be sustained, and the harsh winter of 1939-40 froze Baltic ports and trapped new U-boats in the ice before they ever reached the Atlantic.

• 3. The Happy Time

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  From June to October 1940, U-boat crews were calling it "Die Glückliche Zeit" , the Happy Time. Over 270 Allied ships went to the bottom in those five months. The aces of the U-boat arm became celebrities in Germany: Günther Prien of U-47, Otto Kretschmer, Joachim Schepke, Engelbert Endrass, Victor Oehrn, Heinrich Bleichrodt. What had changed to produce this slaughter was geography. Germany had just conquered France, and the U-boat bases at Brest, Lorient, and La Pallice near La Rochelle were about 450 miles closer to the Atlantic than the old North Sea bases. Boats that once spent days just reaching their hunting grounds could now spend far longer on patrol, effectively doubling the operational size of the U-boat force. The British were simultaneously weakened. Seven destroyers had been lost in the Norwegian campaign, another six at Dunkirk, and a further ten in the Channel and North Sea between May and July. Dozens more were damaged. The same destroyers that should have been escorting convoys were held in the Channel to guard against a German invasion. Wolf packs now pressed the advantage brutally. On the 21st of September 1940, convoy HX 72 of 42 merchantmen was attacked by just four U-boats; eleven ships sank over two nights. In October, the slow convoy SC 7 lost 59% of its ships. The escort for HX 79 was strong on paper , two destroyers, four corvettes, three trawlers, and a minesweeper , yet still lost a quarter of its ships without any U-boat being sunk in return. On the 1st of December, seven German and three Italian submarines caught convoy HX 90 and sent ten ships down. By year's end, two million gross tons of British merchant shipping, representing 13% of the entire fleet, sat under repair and out of action. The most daring commanders, Kretschmer among them, had taken to penetrating the escort screen entirely and attacking from within the columns of merchantmen themselves.

• 4. The Cipher War

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  Beneath the surface battle, a secret contest of equal consequence was underway. Dönitz's wolf pack tactics depended on radio: one U-boat sighting a convoy, transmitting coordinates to headquarters, headquarters vectoring in the rest. The volume of radio traffic was enormous. The Germans encrypted every message using the Enigma cipher machine, which they regarded as unbreakable. The Kriegsmarine's version was even more secure than the Army's or Air Force's version. Its rotors were chosen from a set of eight rather than five, changed every other day, with message settings determined from bigram tables issued to operators. In 1939, most codebreakers at Bletchley Park agreed the naval Enigma was impenetrable. Two dissenters stood out: Frank Birch, head of the German Naval Section, and the mathematician Alan Turing. The physical capture of Enigma rotors during the sinking of a German vessel in February 1940 provided the wiring information the codebreakers needed to begin. On the 9th of May 1941, crew members of a destroyer boarded a stricken U-boat and recovered bigram tables and current Enigma keys. The haul allowed all U-boat traffic to be read for several weeks. Throughout late 1941, Enigma intercepts combined with radio direction-finding allowed the British to plot U-boat patrol lines and route convoys around them. Merchant ship losses dropped by over two-thirds in July 1941. Then, on the 1st of February 1942, the Kriegsmarine switched U-boats to TRITON, a new four-rotor Enigma network. Allied codebreakers went blind. For ten months they could not read the U-boat signals, and wolf packs ravaged convoy after convoy without warning. The darkness lifted only on the 30th of October, when crewmen salvaged Enigma material from a foundering U-boat off Port Said. By December 1942, decrypts were again revealing U-boat positions. But the Germans had meanwhile broken Naval Cipher 3, the code used for Anglo-American communication. Günter Hessler, Dönitz's son-in-law and first staff officer at U-boat Command, later described reading British messages within a day or two, sometimes within hours. Bletchley Park assigned only two people to investigate whether the Germans had cracked the code. After five months they concluded yes. The Admiralty was told in August 1942 but waited until June 1943 to change the codes, a delay that Captain Raymond Dreyer, the British HQ's deputy staff signals officer at Liverpool, later said had directly enabled some of the most successful U-boat pack attacks of the war.

• 5. America's Costly Welcome

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  When the United States entered the war after Pearl Harbor, Dönitz saw an opportunity. He had only five Type IX boats able to reach American coastal waters for Operation Drumbeat, but those five boats found a shocking state of unpreparedness. The American East Coast had no blackout. U-boats surfaced at night and silhouetted merchant ships against the glow of city lights, picking off targets with ease. The first boats reached American waters on the 13th of January 1942; by the time they withdrew on the 6th of February, they had sunk 156,939 tonnes of shipping without losing a single submarine. Admiral Ernest King, Commander-in-Chief of the US Fleet, initially rejected British calls for a coastal blackout or convoy system. His defenders argue the US destroyer fleet was genuinely limited, partly because 50 old destroyers had already been sent to Britain. King also could not legally impose coastal blackouts; civil defence authority lay with the Army. The campaign that followed was devastating. Type IX and Type VII boats, refuelled at sea by Type XIV "Milk Cow" tankers, pressed the assault and eventually sank 397 ships totalling over 2 million tons. U-boats ranged through the Battle of the Caribbean and the Gulf of Mexico, where they effectively closed several US ports. Britain had to build coastal escorts and provide them to America under what amounted to a reverse Lend-Lease. Only when King instituted a convoy system in May 1942 did the killing rate drop sharply , that single move quickly led to the loss of seven U-boats. Meanwhile in 1943, American shipyards were launching over 11 million tons of new merchant shipping per year, replacing losses faster than U-boats could sink them. That industrial fact would prove as decisive as any tactical innovation at sea.

• 6. Technologies That Turned the Tide

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  Between February 1942 and July 1945, about 5,000 naval officers played war games at the Western Approaches Tactical Unit. The battle on the linoleum floor, as many graduates put it, was essential to what came next at sea. In November 1942, Admiral Horton tested a search method called Beta Search in one of those games. Janet Okell and Jean Laidlaw played the escort commanders. Five times running, they sank Admiral Horton's submarine. The weapon developments being tested in those games and at sea were equally ingenious. The Leigh Light, introduced in January 1942, was a powerful searchlight mounted on Wellington bombers and B-24 Liberators. Aircraft using air-to-surface radar would locate a surfaced U-boat, then switch the light on at about one mile from the target. Commanders had less than 25 seconds to react before depth charges arrived. The first confirmed kill using the Leigh Light was U-502, sunk on the 5th of July 1942. The effect on U-boat routine was immediate: the habit of surfacing at night to recharge batteries was largely abandoned as too dangerous, and boats chose daylight instead when enemy aircraft at least could be seen. Hedgehog, a multiple spigot mortar installed on anti-submarine ships from late 1942, solved a different problem. Standard depth charges were rolled from the stern, creating disturbed water that made it hard to regain sonar contact. Hedgehog fired contact-fused bombs ahead of the attacking ship while the target was still within the sonar beam. Because Hedgehog only exploded if it actually struck the submarine, a miss left the water undisturbed and tracking could continue. Its successor, Squid, introduced in late 1943, was a three-barrelled mortar projecting 100-pound charges automatically linked to the latest sonar sets. High-frequency direction-finding (HF/DF, called Huff-Duff) exploited the wolf pack's own radio discipline against it. Fitted to escorts from February 1942, it could pinpoint a U-boat's transmission the instant an operator touched his Morse key, using a sense aerial to suppress the reciprocal bearing , something German radio operators did not believe was technically possible. An escort could then run straight down the precise bearing to attack. The convergence of all these systems, combined with long-range B-24 Liberators finally closing the mid-Atlantic air gap on the 18th of March 1943 when Roosevelt ordered 60 of them transferred from the Pacific, produced a reversal that Dönitz himself acknowledged in plain language: "We had lost the Battle of the Atlantic."

• 7. Black May and Its Reckoning

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  May 1943 entered the memory of the U-boat service as "Black May" , Schwarzer Mai. The turning point had been slow convoy ONS 5, a formation of 43 merchantmen escorted by 16 warships that was attacked by a pack of 30 U-boats in late April and early May. Thirteen merchant ships were lost, but six U-boats were sunk by escorts or Allied aircraft. A storm scattered the convoy, yet the merchantmen reached land-based air cover before the pack could press home a decisive blow. Two weeks later, convoy SC 130 lost no ships at all while at least three U-boats were destroyed. For the entire month of May, 43 U-boats were destroyed, 34 of them in the Atlantic. That figure represented 25% of the U-boat Arm's total operational strength, destroyed in a single month. RAF Coastal Command's second Bay Offensive, which began on the 13th of April with 75 long-range aircraft carrying the new centimetric ASV Mark III radar, caught U-boats in transit through the Bay of Biscay that the German Metox detector could not sense. When Dönitz ordered U-boats to stay surfaced and fight back against aircraft, converting some to "flak boats" with extra anti-aircraft guns, five were sunk and seven forced to abort in May alone. The cost to the Allies in the same period was 58 ships, 34 of them totalling 134,000 tons in the Atlantic. The exchange rate had inverted entirely. German naval spending throughout the war had exceeded what the country spent on every type of ground vehicle combined, including tanks. The submarines those funds built were gone faster than they could be replaced, and with them went the crews who had learned to use them. Captain Frederic John Walker, whose 36th Escort Group had achieved the first clear Allied convoy victory with Convoy HG 76 in December 1941, had by this point made the convoy escort a profession rather than a duty, and the results showed in every engagement he commanded.