Winston Churchill dubbed the Battle of the Atlantic the most important of World War II. It was certainly the longest, lasting nearly seventy months. With substantial losses of ships and men on both sides, the Atlantic struggle became a long conflict—really a series of individual actions—of attrition and steadily improving technologies. Though essentially won by 1943 thanks largely to more effective Allied use of code breaking, radar, and aircraft, actual fighting ranged over the whole six years of World War II.

While the Allies sought to protect vital military trade routes, the aim of the German submarine effort (with the occasional aid of surface raiders and long-range bombers) under Admiral Karl Dönitz was to sink sufficient shipping so as to force Britain out of the war. As the war began, Dönitz had fifty-seven U-boats and only twenty-seven fit for long-range Atlantic service. Though he claimed to need 300 to do the job, he achieved initial successes, including the 3 September 1939 sinking of the British passenger liner Athenia.

The U-boat menace in the Atlantic expanded after the German occupation of Norway and France in early 1940, which gave the submarines new coastal bases closer to major shipping lanes. U-boats could now make speedy entry into the Atlantic and interrupt Allied convoy lanes. This inaugurated the so-called happy time when Dönitz utilized Enigma-coded radio communications to direct U-boat wolf packs to more effectively attack the convoys. As the number of Atlantic U-boats increased in 1941, the monthly tonnage of lost Allied ships soared—to a total of more than two million tons in the Atlantic over the course of the war. Some early convoys lost two-thirds of their ships.

The British hoped to deal with the U-boat problem through the use of convoy escorts equipped with depth charges directed toward targeted U-boats by the Allied Submarine Detection Investigating Committee (ASDIC) device or sonar. But ASDIC was not a reliable means of identifying enemy vessels on or under the sea. The convoy system, initiated slowly in the first months of the war, was hampered until 1943 by a lack of sufficient escort vessels and long-range patrol aircraft.

With ASDIC failing to thwart the German U-boats, the British began to devote intensive effort to solve the Enigma codes. In the spring of 1941, Enigma code machinery and codebooks were captured from several German vessels, giving British code breakers a huge leap forward. When the Germans changed their codes in February 1942, however, conditions in the Atlantic deteriorated for the British once again and losses rose as Britain could not read German communications for the rest of the year. U-boats enjoyed another happy time, made worse for the Allies when the U.S. Navy initially resisted convoying ships. Further, for the first six months of 1942, American seaboard cities did not adopt evening blackouts, thus silhouetting vessels against the shore and making them easy targets. German code breakers were also able to read some Allied convoy codes. Resulting shipping losses in the western Atlantic rose to record levels and the U-boat fleet peaked at over 200.

The eventual demise of the German submarine threat began after March 1943. Britain once again penetrated the improved German codes and as a result could reroute convoys around known submarine wolf packs. Also contributing were the more effective application of radar and ASDIC, the improvement of antisubmarine vessels (and more of them), more reliable depth charges, and the growing use of patrolling aircraft that could read all areas of the Atlantic. In addition, Allied bombing of U-boat construction sites and bases became more effective. In the end the Allies’ ability to replace losses faster than Germany could destroy ships tipped the scales.

Germany’s use of acoustic torpedoes, better radar detection, and antiaircraft weaponry made little overall difference. Nor did, in the final year of the war, snorkel breathing apparatus allowing submarines to stay submerged for extended periods of time. Despite their responsibility for 70 percent of Allied ship losses, Germany’s own submarine losses rose sharply in 1943–1944, and by the end of the war more than 500 U-boats had been sunk during the Atlantic engagement.

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Improved communications allowed political or military leaders to micromanage distant battles, a temptation to which Hitler increasingly succumbed as the war turned against Germany. His orders were sent through the huge underground Zossen site near Berlin, all of them coded by the Enigma or more advanced devices—and by the end of the war, most were being read in real time (as Ultra) by the Allies. Although all sides relied on machine encryption to protect their communications, the British Government Code & Cipher School at Bletchley Park and American cryptanalysts at Arlington Hall and Nebraska Avenue developed techniques to break codes (aided by captured codebooks) and thus read enemy messages almost as quickly as their intended recipients. Alan Turing, John Tiltman, Gordon Welchman, and others worked at Bletchley Park to develop early analog computers to assist in the growing code-breaking task. The ability to read enemy codes helped in several highly successful Allied deception efforts to mislead enemy commanders.

Indeed, the code-breaking advantage of the Allies (in one of the closest held secrets during, and for decades after, the war) had a huge impact on the course of the war, from the eventual winning of the Battle of the Atlantic against German U-boats to placing American forces in the right place to defeat the Japanese navy at the Battle of Midway. Careful monitoring and analysis of enemy radio transmissions, or signals intelligence, brought vital information to the Allies. On the other hand, the American and British electric cipher machine (SIGABA and Typex) equipment and the SIGSALY system used by Prime Minister Churchill and President Roosevelt to talk by telephone across the Atlantic, each of them perfected during the war, could be operated by hastily trained personnel and proved invulnerable to enemy code-breaking efforts. Code-breaking abilities were very closely held, and many field commanders did not know the derivation of information provided to them (which did not help them believe what they were told).

Sources

Blair, Clay. 1996, 1998. Hitler’s U-Boat War. 2 vols. New York: Random House. Kahn, David. 1991. Seizing the Enigma: The Race to Break the German U-Boat Codes, 1939–1943.

Boston: Houghton-Mifflin. Sarty, Roger. 1997. “The Limits of Ultra: The Schnorkel U-Boat Offensive Against North America, November 1944–January 1945.” Intelligence and National Security 12 (2): 44–68.

Showell, Jak P. Mallmann. 2000. Enigma U-Boats: Breaking the Code. Annapolis, MD: Naval Institute Press.

Syrett, David. 1994. The Defeat of the GermanU-Boats: The Battle of the Atlantic. Columbia: University of South Carolina Press.

Williams, Andrew. 2003. The Battle of the Atlantic. New York, Basic Books.