The Zimmermann Telegram

World War I began in 1914 as a fight among several European nations, while the United States pursued a policy of non-intervention. In fact, Woodrow Wilson was reelected President largely because “He kept us out of war”. But as the war unfolded in Europe, an intercepted telegram sent by the German Foreign Secretary, Arthur Zimmermann, to the Mexican government inflamed the U.S. public opinion and was one of the main reasons for the entry of the U.S. into WWI. This is the story of the encrypted telegram that changed the last century.


Area Mexico ceded to the United States in 1848, by [Kballen] via Wikimedia Commons

Germany was actively seeking to promote a war between Mexico and the U.S., in the hope to create a military distraction and avoid the entry of the U.S. into the war. After several unsuccessful attempts to spark a conflict, the Germans decided to offer Venustiano Carranza, the new Mexican president, the recovery of the lost territories of Texas, New Mexico and Arizona in case of a German victory. (California was not included, maybe because the Germans thought it would be an irresistible offer for the Japanese, in case they also wanted to declare war on the U.S.)

But how could the Germans convey this offer to the Mexicans? Three options were considered.  The first was to communicate the proposal to the Mexican ambassador in Berlin. The second, send the message via the Deutschland submarine that would land somewhere in the U.S. and then deliver it to the German embassy, to be forwarded on to the Mexicans. The third option was to send the proposal in a coded telegram using the best cryptographic techniques available.

The Mexican ambassador in Berlin was not trusted by Germany, so this option was considered too insecure. The second option offered the most secure method, but it was also the slowest. Therefore, the Germans went for the third option and on January 19, 1917, the German ambassador to Mexico, Heinrich von Eckardt, received the coded telegram from Zimmermann. The decoded text is as follows:

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


A military commission was created by Mexican president Venustiano Carranza to study the German proposal. The commission recommended to refuse it, based on several reasons. First there was little chance that Mexico could win a war with the U.S. Second, a war against the U.S. would endanger Mexican relations with other nations. And third, there were serious doubts that Germany will keep the promise of “generous financial support”.

British intelligence intercepted and decoded the telegram, and released it to the U.S. on Feburary 24. That same day, President Wilson read the message, and went public with it on March 1st. The reaction of the American public proved to be just what was needed to change the non-intervention policy. The threat of a possible combination of Mexico, Japan, and Germany represented a nightmare. On March 20, President Wilson met with his cabinet, and found it in favor of entering the war. The American congress approved the declaration of war on Germany and its allies on April 6, with just one vote against.


ztel2Decoding the Zimmermann telegram is perhaps the most significant intelligence triumph for Britain during World War I. The telegram was intercepted and analyzed in the ultra-secret Room 40, a section of the British Admiralty for cryptanalysis efforts during WWI.

The Zimmerman telegram was coded using cypher 0075, a two-part code of 10,000 words and phrases numbered 0000 to 9999. In order to avoid frequency analysis, the numbers were randomly chosen, and additionally, each letter was individually coded using simple substitution. Of course, the cypher is secure as long as the code book remains secret. An older code, named 13040, was already been cracked by the British, but the newer code 0075 was considered secure. Code 13040 had about  25,000 plaintext elements and 75,000 code numbers. For instance, here are some examples from the codebook:
Februar 13605
fest 13732
finanzielle 13850
folgender 13918
Frieden 17142
Friedenschluss 17149
führung 17166
Ganz geheim 17214
Gebeit 17388
geheim 4377
Gemeinsame 4458

However, the Germans underestimated the British codebreakers. The telegram was first sent from Berlin to Washington on January 16 (using the U.S. diplomatic lines), intercepted by the British, and was partially decrypted by the people in Room 40. Even with an incomplete message, the intentions of Germany were evident. The British wanted to pass this information to the U.S., but to do so would reveal the eavesdropping on U.S. cables. At the same time, the Germans would realize that their new 0075 code had been broken.

In order to prove the telegram’s authenticity to the U.S. without German suspicion of code breaking, and without disclosing the fact that the British intelligence were spying on the U.S. cables, the British would tell the U.S. that the telegram was obtained by bribing a telegraph company employee in Mexico (the telegram was sent from the Germany embassy in Washington to Mexico). Moreover, the telegram sent to Mexico was coded using the older 13040 cipher, so, at worst, the Germans would think that the 13040 code was cracked.

In the end, Germany sealed its own fate, providing Britain with a way to push the U.S. into the war. President Wilson had said that it would be a “crime against civilisation” to lead his nation to war, but a single breakthrough by the codebreakers in Room 40 forced him to change his mind. It was a great triumph of cryptography that drew the United States into the war, and changed the course of history.

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