On the evening of September 3, 1939, the passenger ship SS Athenia was struck by a devastating explosion on her port side. She had been torpedoed by the German submarine U-30—and the German Navy had fired the first shot in what would become the longest continuous battle of World War II: the Battle of the Atlantic.

The vast battlefield stretched from the icy waters off Greenland to the sunlit Caribbean, and the conflict was never merely a duel between ships and submarines. Aircraft, intelligence, and technology would all play vital roles, redefining naval warfare forever.

As an island nation, Great Britain relied entirely on merchant shipping for trade and supplies—a vulnerability her enemies had long recognized. Germany’s aim was to sever those supply routes and bring Britain to her knees. At the helm of the U-boat force was Admiral Karl Dönitz, a clever tactician who constantly refined his strategy to maximize the effectiveness of his small fleet. At the outbreak of war, he had only 57 submarines—not enough for a wide-ranging attack, but more than adequate to inflict significant losses on British merchant shipping in the early stages.

Dönitz developed the concept of the wolf pack: once intelligence identified a convoy, he would order as many U-boats as possible to lie in its path. Sometimes a dozen or more submarines would converge, overwhelming the escorts. “The wolf pack was my own invention,” Dönitz later claimed, though such methods had been used in World War I. He simply took the idea much further.

U-boats proved most effective when attacking at night. During daylight they shadowed the convoy on the surface; as darkness fell, they would submerge and find a suitable firing position. Braver commanders would slip inside the convoy itself, launching salvos of torpedoes at clustered targets. A single spread might hit two or three ships before the escorts could react.

The Allies, meanwhile, fought to get merchant ships safely into British ports. Convoys were organized—groups of 40 to 60 ships, either fast (about 15 knots) or slow (around 10 knots). Ideally they were escorted, but such was the shortage of escorts at the start of the war that most convoys had naval protection only as far as 12 degrees West. Beyond that line, they were on their own.

Both sides soon realized that intelligence was critical. The British turned to Bletchley Park, where the Government Code and Cipher School worked to break the German Enigma codes. The information was passed to the Submarine Tracking Room, where convoys could be rerouted away from known U-boat positions. But there was a problem: Bletchley Park could not instantly decode messages without access to the Enigma machine itself.

In May 1941, Convoy OB 318 was attacked by seven U-boats, including U-110, commanded by Julius Lemp—the man who had sunk the Athenia. U-110 was badly damaged and forced to surface. Lemp ordered the crew to abandon and scuttle the boat, but the scuttling failed. A British boarding party from HMS Bulldog went aboard and discovered that neither the Enigma machine nor its code books had been destroyed. The Admiralty was overjoyed. By clever subterfuge, the British convinced the Germans that U-110 had sunk with all her secrets.

But the Allies still had to train people to use the technology and interpret the information—time they did not have. There was also the temptation to use Enigma intelligence immediately to hunt U-boats. While this could have saved ships, it would have told the Germans that their codes had been cracked. The information had to be used very carefully.

Dönitz, meanwhile, knew from the start that he lacked the resources for an effective submarine war. Hitler was initially reluctant to introduce unrestricted submarine warfare, fearing the impact on neutral countries. He also insisted that U-boats follow prize rules—surfacing to confirm a target was legitimate—which made them vulnerable to counterattack. Dönitz was also far from convinced that his torpedoes were reliable, as failures against HMS Hood demonstrated.

But one thing his World War I experience had taught him was that all that mattered was tonnage. His submarines needed to sink Allied merchant ships at a greater rate than they could be replaced. If that happened, Britain could be starved into surrender.

Dönitz convinced his commander-in-chief, Grand Admiral Raeder, that 300 U-boats were needed to guarantee success. Hitler appreciated the argument but stopped short of giving the U-boat force top priority. The German Army still lay at the root of his belief in victory, and Göring argued convincingly for more resources for his Air Force. It was well into 1940 before Dönitz began to receive new submarines in the numbers he needed.

The first phase of the battle was inconclusive. The Germans did not have enough submarines to be truly effective, and the British lacked escorts. But if anything, it was the British who had won the first round by a very narrow margin: 17 U-boats had been sunk, and the convoys were getting through mostly unscathed.

That situation was not to last. The capture of the French ports following the fall of France was vital to the U-boat campaign. On June 23, 1940, a convoy of trucks left Wilhelmshaven loaded with submarine spares, weapons, and equipment, bound for Lorient in southern Brittany, where the first French U-boat base was being established. Soon massive concrete pens were built to protect U-boats from aerial bombing. From these bases, U-boats could sail straight into the Atlantic without the long voyage from Germany, staying on patrol for longer and allowing more submarines to be available at any one time.

Dönitz returned to his wolf-pack idea. Six boats were placed under the command of Günther Prien—the man who sank the Royal Oak—and five under Hans-Rudolf Rösing. The concept was good, but there were still problems of communication, and both target convoys arrived unscathed. Dönitz then decided to attack independent ships, and during June 1940 his U-boats sank 58 victims—only a fifth of them in convoy.

The British were worried. By summer 1940, the fear of invasion was diverting escorts to the English Channel, reducing convoy protection in some cases to just one ship. The fall of France also enabled German long-range bombers to attack British shipping in the southwest approaches. So the British moved their convoy routes to the northwest—but this made them vulnerable to short-range U-boats operating from bases in Norway.

The British did have one advantage: continued rivalry between Dönitz and Göring meant there were never enough German long-range aircraft for reconnaissance. The Luftwaffe was heavily engaged in the Battle of Britain, and Göring had little time for the Atlantic Campaign, which he brushed off as a sideshow.

Most of the cards, however, still lay with the Germans. U-boats based in France could now operate as far west as 25 degrees West; British escorts could only reach 17 degrees, and Coastal Command’s Sunderlands could provide cover out to 15 degrees. The British had to find an answer.

One solution was to increase the number of escorts. In the long term they could be built, but in the short term Churchill would have to persuade Roosevelt to lend Britain 40 old destroyers. They were ill-equipped and had poor seakeeping qualities, but they came throughout the autumn of 1940.

The U-boats gained the upper hand, attacking on the surface at night. They started to cause losses which the British found hard to sustain. Escort tactics were poor, and the Germans were learning quickly. Convoys were attacked at night; the U-boats would shadow by day and attack again the next night. Some very successful U-boat commanders, such as Prien and Otto Kretschmer, became household names back in Germany and were accorded the status of aces—part of Dönitz’s astute public relations campaign to convince the public of U-boat success and force the Nazi leadership to give the force more resources.

In October 1940, Convoy SC 7 left Halifax for Liverpool and lost 17 ships out of 35. In the same month, Convoy HX 79 lost 14 ships, and HX 79A lost seven. In less than a week, 38 merchant ships had been sunk by U-boats. No wonder the Germans called this period “the Happy Time.”

Britain responded in three ways: escorts were released from anti-submarine duties and given radar; more aircraft were provided as older bombers were released to Coastal Command; and tactically, escorts were told never to leave the convoy unprotected while chasing contacts. Slowly these measures began to take effect, but the reduction in losses in late 1940 was more due to submarine unserviceability and bad weather than new British tactics.

To compensate for the bad weather, the U-boats moved south, where they found rich pickings in the shipping off the West African coast where escorts were non-existent. During the winter of 1940–41, the Allies started to fight back. A big reorganization in command and control, and better use of joint intelligence, started to have an impact. Losses were still happening, but proportionately they were less than in the desperate autumn of 1940.

Even the aces began to encounter more resolute defenses. In March 1941, Prien himself was lost when attacking Convoy OB 293. Kretschmer surrendered after being forced to surface and scuttle U-99. A third ace, Joachim Schepke, was crushed between the bow of HMS Vanoc and his own periscope as his U-boat was rammed and sunk. These losses were a serious blow to Dönitz—five U-boats lost in three weeks for the exchange rate of 15 Allied merchant ships. This loss rate was unsustainable, and Dönitz was still suffering from a lack of available submarines.

He decided to change tactics. In April 1941, he moved his nine operational deep-sea U-boats further west to establish a patrol line at 30 degrees West. This soon reaped dividends when Convoy SC 26 lost 11 ships; the cost to the Germans was one U-boat. This forced the British to develop facilities on Iceland, particularly the airfields. Sunderlands and Hudsons were sent to the island and helped push the protection screen further west, but there were still not enough escorts or long-range aircraft.

The capture of the Enigma machine offered the Allies some respite, but it would be some time before the full impact could be assessed and the information used. This and other technical advances needed time, operators, and assessors.

However, one advantage was now beginning to produce results: the increasingly obvious support for the British by the Americans. As early as October 1939, the Americans had declared a security zone around the US coastline. On April 10, 1941, President Roosevelt announced that the zone would be moved from 60 degrees West to 26 degrees West, and that British warships could be repaired in American dockyards. This blatant abuse of neutrality rules meant the security zone now stretched 2,300 nautical miles across the Atlantic and included the Azores.

It was inevitable that U-boats would become more confused about where they could operate and who the enemy was, especially when American warships were seen escorting British convoys within the neutrality zone. Mistakes occurred: American warships were attacked, and the USS Reuben James was sunk with heavy loss of life. More American merchant ships were also attacked—usually by accident, but in some cases deliberately. Hitler had to issue a direct command to stop German U-boats targeting Americans; he was still desperately worried about provoking the United States into war.

By the summer of 1941, Dönitz was infuriated at what he saw as blatant interference by a neutral state. He was also concerned at increasing losses among German supply ships, which were maintaining and resupplying submarines in distant waters. These losses were the first tangible benefits of Bletchley Park’s cracking of the German codes.

In June, things became even worse for the Germans as Roosevelt ordered American forces to occupy Iceland. Within a few months, Iceland had become a well-equipped and well-defended Allied base instead of merely hosting a few RAF Coastal Command squadrons. But Hitler still refused to react.

In August 1941, a U-boat was captured almost fully intact by a combination of surface escorts and RAF Coastal Command aircraft based in Iceland. Unfortunately, the crew of U-570 managed to destroy their secret equipment and documents before being boarded. After examination, the U-boat was taken into the Royal Navy and named HMS Graph.

By the end of 1941, the situation was improving significantly for the British. Dönitz was still chronically short of U-boats; Allied anti-submarine equipment and experience were improving; and growing American involvement was having an effect. At the same time, Hitler ordered Dönitz to send his valuable submarines to support the campaigns against the Russians and in the Mediterranean. In the Mediterranean, some very visible successes were achieved: the legendary aircraft carrier Ark Royal was sunk, and the old battleship Barham was destroyed most spectacularly by U-331. But Dönitz knew these would not win the war.

The Japanese attacked Pearl Harbor on December 7, 1941, but the United States had been in conflict with Germany for some time before Germany officially declared war on December 11. The chief of Naval Operations, Admiral Harold Stark, admitted this when he said, “We are at war in October 1941,” following the sinking of the Reuben James.

With the official declaration, the situation in the North Atlantic became much clearer, and Dönitz, who knew the implications of US entry, could at least recognize who the enemy was. But he realized that in the long term, American war production would be crucial to his tonnage war. He calculated that just over 800,000 tons would have to be sunk each month if his campaign was to succeed. The figures for the last six months of 1941 did not give him much cause for optimism: only 120,000 tons were being sunk each month.

At last, U-boat production was beginning to pick up. Dönitz had 91 U-boats available in January 1942; by July this had increased to 138. This still meant he had nowhere near enough—many boats were in maintenance, on training duties, or working up to operational fitness. So on January 1, 1942, Dönitz was only able to deploy six boats against the United States. This did not stop him. He began the campaign almost immediately. He called it Operation Paukenschlag—Operation Drumbeat.

He told the six commanders to avoid convoys and operate independently. Attempts were made to track the submarines, but bad weather and operational difficulties meant the information available to the Allies was inconclusive. On January 12, U-123 found the first victim. In all, the operation accounted for 41 merchant ships for no German losses. It was the second Happy Time.

The Americans, despite their already significant experience, were totally unprepared. Many of their escorts had been sent to the Pacific; the ones that remained were poorly equipped and manned by inexperienced crews. Ironically, the best anti-submarine units available were seven US Coast Guard cutters. It was perhaps now a source of regret that many of the ships the Americans could have used had already been sent to Britain under Lend-Lease.

Admiral King would not produce convoys until he had the ships to escort them. This was against all the advice of the British, who had experienced similar problems two years earlier. But King refused to accept British advice. Nor did the American Eastern Sea Frontier command have the aircraft it needed; these had been sucked into fleet protection, and many were under the control of the Army Air Force, with whom the Navy did not work well.

To help, the British temporarily lent the Americans 10 corvettes and 24 trawlers until US forces became available. The Americans realized quickly that things would have to improve and set about producing new command and control systems to enable all parties to work together more closely. At last they began to learn the lessons of the British experience. They also began a huge construction program.

But the U-boats continued their deadly work. They moved south into the Caribbean and the Gulf of Mexico. They even surfaced and used their deck guns to shell oil installations ashore. The American reaction was poor; even when U-boats were located, inexperienced escort crews lost them or failed to press home their attacks. On one occasion, a merchant ship opened fire on an American destroyer believing her to be a U-boat; the destroyer’s captain and two sailors were killed.

The British tried and tried to convince Admiral King to introduce convoys, but still he refused, believing that a convoy not properly escorted would be a bigger liability than merchant ships sailing individually. The British also tried to get the Americans to shift priorities from the Pacific to the Atlantic.

While this was going on, the Germans continued to exact a heavy toll off the Eastern Seaboard of the United States, where ships were often sunk within visual distance of the coast. No blackout had been introduced, making it much easier for U-boats to spot merchant ships silhouetted against the lights of the cities.

By March, things were beginning to improve as new escorts began to arrive, but it was still May 1942 before the Americans began to convoy ships down the east coast and into the Gulf. Once this happened, the number of easy targets for the U-boats began to decline, and so Dönitz turned his attention back to the North Atlantic convoys and reintroduced his wolf packs.

He still had to attain his required number of sinkings to keep his tonnage war on track. Despite improvements to American anti-submarine capability, the demands of the Pacific drew more and more resources there, leaving the Battle of the Atlantic to the British and the Canadians. A mid-ocean escort force was formed based in Newfoundland, which would take convoys straight from Newfoundland to the coast of Ireland. More new escorts were entering service, including the first of the new purpose-built “super corvettes” which became known as frigates.

But as new ships entered the fray, their lack of experience became critical. This was especially true in the case of the Royal Canadian Navy, which had expanded from almost nothing to provide 33% of the escort vessels in a matter of 18 months. It was not surprising that the Canadians were not as competent as they were expected to be.

The convoy battle was rejoined with a new fervor, and the Atlantic war was about to reach its most critical phase.

The climax came in the early spring of 1943. Dönitz threw his wolf packs against convoys SC 121, SC 122, HX 228 and HX 229. Twenty-two ships went down for the loss of a single U-boat. For a moment, it seemed the Atlantic lifeline could be severed.

But operational analysis told a different story: more escorts, more long-range aircraft, and better intelligence were finally converging. In April, Convoy ONS 5 met the largest wolf pack of the war—40 boats. Eight U-boats were sunk; the attack was called off.

Then came SC 130. Thirty-three U-boats attacked, and not one merchant ship was lost—five U-boats were destroyed, one carrying Dönitz’s own son.

On May 24, Dönitz ordered his boats out of the North Atlantic. “We had lost the battle,” he later admitted. The U-boats fought on to the last day, but never again threatened the Atlantic.

Was the Battle of the Atlantic won by technology, by courage—or simply by the sheer weight of Allied shipyards? Tell us what you think in the comments.