The Zimmerman Telegram: The Time When British Codebreakers Changed The Course Of History By Deciphering A German Message, Causing The United States To Enter World War I

In order to understand modern cryptography it is important to understand the history of cryptography. One of the most important incidents involving cryptography occurred in 1917 when British codebreakers intercepted an encrypted German telegram, and this is known as the Zimmerman Telegram. This incident proved that cryptography, and the ability to decipher code, can change the course of history. 

By 1917 France and Britain were locked into bitter trench warfare with Germany after two and a half years of continuous war. France and Britain saw heavier casualties than Germany, since they mounted several offensives that failed, while the Germans only did one major offensive, which also failed. Morale was dropping for France and Britain as countless men died for seemingly no gain. France and Britain desperately needed the United States to come to their aid in order to turn the tide of the war, but the United States remained staunchly neutral. 

That being said, the United States continued to trade freely with Britain, France, and Germany. The Germans recognized that if they could cut off supplies from the United States to Britain and France it could expedite a German victory. This led Germany to attempt to seduce Mexico into declaring war on the United States, which would prevent American troops from ever fighting the war in Europe, and would also cut off American supplies. This is where the Zimmerman Telegram comes in.

Since the United States wanted to act as a mediator to end the war in Europe, the United States allowed German diplomats to send messages via the American transatlantic cable to Washington DC. 

Germans were allowed to send messages across this transatlantic telegram cable via Copenhagen, and the Germans believed from there the cable was completely submarine, which would prevent spying. Also, Germany believed France and Britain would not tap into the American cable since that could cause an explosive international incident. 

The Germans were unaware that the American transatlantic telegram cable actually crossed Britain from Newcastle to Cornwall, and a relay station in Britain was used to amplify the signal so it could reach Washington DC. Britain intercepted the German and American messages crossing this cable, and sent the messages to the codebreakers in Room 40

Originally Germany was not supposed to send encrypted messages over the American transatlantic telegram cable, but the Germans persuaded the United States ambassador to accept the Zimmerman Telegram in coded form on January 16 1917.

The version of the Zimmerman Telegram which was intercepted physically in Mexico, using a lesser code than the higher code that was originally deciphered by Room 40

The British intercepted the message immediately, and Room 40 codebreakers ‘Dilly’ and Nigel de Grey had partially deciphered the telegram by the next day. Of great use was the fact that the British has captured the cipher for German Code 13040 during the Mesopotamian Campaign of World War I, and naval cipher 0075 from the wreckage of the SMS Magdeburg, and these ciphers were passed on to Room 40 before the Zimmerman Telegram. 

A cipher is essentially the key to an encrypted code, and can be used to encrypt or decrypt messages. 

A previously confidential study from the War Department of the United States titled “Studies In German Diplomatic Code Employed During The World War” gives a general idea of the encryption used in the Zimmerman Telegram. 

The German code consisted of a trinumeral code used to date messages, a set of miscellaneous common phrases, a common vocabulary that spanned across hundreds of pages with a hundred words per page corresponding to numerical values, code for thousands of geographical and proper names, and grammatical directions to determine the tense of verbs and the degree of adjectives. 

Codebreakers went about breaking this code by finding convergences of the numerical and alphabetical sequence, which usually yielded a cipher for a few words. From there the codebreaker could decrypt the rest of the message. Additionally, the capture of partial German ciphers previously during the war gave the codebreakers something to start with. 

The Germans sometimes switched up their code, but this required a manual revamp of the entire cipher which consists of hundreds of pages, so changing the code was infrequent. This, and the fact that the code is based on the dictionary, are top reasons as to why the Zimmerman Telegram was deciphered.

Original notes from the Room 40 codebreakers, showing the relationship between numbers and words in the German code. Once the codebreakers had these inroads into the code they were able to decipher the rest of the message.

When the Zimmerman Telegram was finally decoded the results were shocking: “We intend to begin on the first of February unrestricted submarine warfare. We shall endeavor in spite of this to keep the United States of America neutral. In the event of this not succeeding, we make Mexico a proposal of alliance on the following basis: make war together, make peace together, generous financial support and an understanding on our part that Mexico is to reconquer the lost territory in Texas, New Mexico, and Arizona. The settlement in detail is left to you. You will inform the President of the above most secretly as soon as the outbreak of war with the United States of America is certain, and add the suggestion that he should, on his own initiative, invite Japan to immediate adherence and at the same time mediate between Japan and ourselves. Please call the President’s attention to the fact that the ruthless employment of our submarines now offers the prospect of compelling England in a few months to make peace. Signed, ZIMMERMANN”

Essentially, Germany was to begin unrestricted submarine warfare in the Atlantic, which would severely disrupt trade and possibly bring America into the war. Indeed, when the submarine warfare began exactly as stated in the message on February 1, America cut off diplomatic ties with Germany. In order to prevent America from entering the war, Germany wanted to distract America via a war in Mexico, and as payment for that Germany offered Mexico large swaths of the American Southwest. 

Britain did not immediately send this telegram to the United States, since Britain had to obfuscate the fact that they had illegally been intercepting all messages on the American transatlantic telegram cable, and that they had broken the German code, since Germany would then change the code. 

Britain knew that the telegram would be sent by the German ambassador to the United States, Johann Heinrich von Bernstorff, to Mexico via Western Union. A British agent intercepted a copy of the Western Union version of the telegram, which was in German Code 13040, a lesser code than the code originally broken at Room 40. The British then revealed the message to the United States, and used the Mexican telegram as proof, claiming that it was a lucky intercept rather than espionage on American telegram cables. 

On February 18 the American ambassador to Britain was informed of the Zimmerman Telegram, and by April 5 the United States declared war on Germany, a little over a month after the Zimmerman Telegram was given to the United States. 

Thus, this is a case where the ability to decrypt encrypted messages changed the course of global history, ultimately leading to the defeat of Germany in World War I, which set the stage for World War II.