The Battle of the Atlantic was a life-and-death struggle between the German Kriegsmarine and the Allied navies that was fought for control of Britain’s lifeline to its empire and to the United States.
Although the Allies triumphed in the end, it was no sure thing. The Germans came very close to knocking the British out of the war. If that had happened, America would have had to fight a two-front war without the huge contributions of the British. Many factors contributed to the eventual outcome. In the end, Allied technical and logistical superiority proved decisive. Perhaps the most intriguing aspect of the long battle was the role played by signals intelligence (SIGINT) on both sides. It is widely known that British codebreakers were ultimately successful in reading the German naval codes. It is less well known that the German codebreakers were also successful for much of the war in reading the British Navy’s coded radio message traffic.
By 1938, the German Navy was deep into planning for war with England. The German naval staff knew that Britain was one of the world’s foremost naval powers. It also knew that Britain could not fight for long without substantial imports of food and raw materials for its war effort, as Britain was not self-sustaining in agricultural products, steel, oil, and other key resources even in peacetime. Its need for these resources would increase substantially during wartime.
The German naval staff, therefore, concluded that a total economic blockade of Britain offered the best hope for victory. The German naval strategy, relying heavily on her fleet of submarines, would therefore focus on a continuous, full-scale campaign against merchant shipping in order to strangle British trade, eliminate its ability to fight, and force a negotiated surrender.
Likewise, the British naval strategy was to secure its own sea lines of communications while denying Germany vital imports of raw materials and food. The British believed that their own strong surface fleet, combined with those of their allies such as France, would prevail against Germany’s fledgling fleet of surface capital ships and the larger, more powerful fleets of her Axis allies––Italy and Japan.
With World War I fresh in their minds, the British understood the importance of Germany’s submarine threat. However, they believed that armed convoys and the development of acoustic underwater detection systems would allow them to overcome this threat. While the British anticipated a continuing struggle with the Axis fleets, they believed they and their allies would eventually prevail.
Many naval and air resources, technology, tactics, and procedures were eventually employed by both sides to implement their respective strategies. However, intelligence based on decryption of enemy Morse code radio transmissions by both sides played a key role in the eventual outcome of the battle. It is a fascinating story of move and countermove. It pitted military personnel and university scientists in a continuing battle of wits.
The SIGINT battle also led to the development of the first programmable electronic computer. It was used by the British late in the war to break the ciphers used by German Army and Air Force radio teletype equipment. Without the use of radio for the centralized command and control of both Allied and German forces, there could be no radio-derived signals intelligence. Without specialized mechanical sorting machines, invented at the outbreak of war and refined as the war progressed, there could be no code-breaking success. The SIGINT battle between the Germans and the Allies was an artifact of early 20th-century technology. It was the cyber warfare of its day.
The British SIGINT operation was eventually code-named Ultra. It involved complex, secret activities to intercept, decrypt, and exploit German high-frequency radio Morse code and teletype traffic. A network of British radio intercept sites, called the Y Service, spread across the country and manually recorded enemy coded radio signals and forwarded the unreadable messages to a secret center. Here the messages were decrypted and forwarded to various intelligence centers and headquarters to aid in the conduct of the war. As long as the Germans had high confidence in their ciphers, the opportunity for the British to peek inside the German war machine continued.
The U-Boat vs the Surface Fleet
For its own part, the German Kriegsmarine understood the importance of code breaking and the vital intelligence it could produce. As early as the mid-1930s, the German Navy Signal Intelligence Unit had broken some of the Royal Navy’s most important codes.
The Beobachter Dienst, abbreviated B-Dienst, translates into English as the Observer Service; it was the German naval radio SIGINT service. Besides radio direction-finding and traffic analysis, the B-Dienst had within it a code-breaking activity. The German radio intelligence system also included a network of intercept sites that forwarded encrypted British radio messages to B-Dienst headquarters in Berlin, where messages were decrypted and exploited for intelligence to support military operations.
To implement the Kriegsmarine’s grand strategy of strangling Britain, Germany would need a fleet that could defeat the Royal Navy’s ability to defend Britain’s sea lines of communications. This was a tall order since Britain was a world naval power. Its plan envisioned a navy of many large capital ships––battleships, cruisers, and even aircraft carriers. In early 1939, Hitler approved the Kriegsmarine’s ship-building plans for a world-class fleet.
The commander of the German Navy’s submarine fleet, Admiral Karl Dönitz, was disappointed with the Kriegsmarine’s plan. He was sure that the only way Germany could succeed in defeating Britain was through submarine warfare. Dönitz believed that the German Unterseeboot, or U-boat, operating in large packs, could sink more British cargo and tanker ships than the British Empire could build and sustain.
He also believed that U-boats could operate undetected and strike with impunity. He knew that submarines were more easily built and operated than a large surface fleet of capital ships. In Dönitz’s view, top priority had to be given to developing a fleet of 300 ocean-going U-boats. The Kriegsmarine staff did not share Dönitz’s view and they persisted with their plans for a grand surface fleet.
The Enigma and the Bombe
While the Kriegsmarine pursued its shipbuilding program, its communications staff also sought to field an encrypted radio communications system that would be up to the task of commanding and controlling a large, ocean-going fleet at war. The encryption machines used by German forces evolved from a commercial encryption device called “Enigma.” It was first marketed in 1923 as a way to protect companies from industrial spying. The German military recognized its superior potential for protecting military communications; it began purchasing the early version in 1926. As the German military began to employ Enigma, neighboring Poland became concerned. The Poles were aware of German efforts to expand their military beyond the limits set by the Versailles Treaty. Their concern was heightened by the complex communications encryption the Germans were now using.
The German Enigma machine had the appearance of a large typewriter, with an alphabetical keyboard. However, it also had another alphabet on the top of the machine. As one letter was typed on the keyboard, another, different letter would light up on the top of the machine. Pressure on the keyboard letter created an electrical path that caused the letter on the top to light. The electrical path flowed through a complex arrangement of wiring that had an almost limitless number of possible outcomes.
There were two sources of the complexity. The first was a series of three rotating wheels. Each wheel had a different arrangement of internal wiring. When a letter was pressed on the keyboard, an electrical path was created through each wheel and to the next wheel in turn. The path then passed through a reflector that routed it back through the three wheels again, but following a different path through them.
After striking a key, the electrical path went through each of the three wheels, through the reflector, and then by a different path back through the three wheels, and then to a small lamp that lit up a letter. The wheels were not fixed; each had 26 equally spaced contacts. Each time a key was struck, one or more of the wheels rotated. If striking a key one time lit a letter, striking the same key again would light a different letter. In addition, the order of the wheels could be varied when the machine was initially set up. Furthermore, there were electrical plugholes in the front of the machine. Plugging in jumper cables between different sets of plugs further changed the electrical path between the keyboard and the key lights.
To decode a message, the cipher clerks needed not only an Enigma machine, but also had to know how it was set at the transmitting site. The clerk would need to know which wheels to use, the order in which they were to be installed, the combination of jumper cable settings to use, and the original position of the wheels. These were called the encryption keys. Once the machine was set up properly, the clerk only needed to type the characters of the received message on the keyboard while an assistant recorded the letter that lit up as each key was depressed. When finished, the clerk would have the complete German-language message.