On This Day — The Zimmermann Telegram Is Intercepted (January 16 1917)

“No account of the stirring episodes leading up to our entry into the World War can be considered complete without at least a reference to the one in which the Zimmermann telegram played the leading role.”
War Department Office of the Chief Signal Officer
(1938 study)


January 16 2023 — On January 16 1917, British code breakers intercepted an encrypted message from  German foreign secretary Arthur Zimmermann intended for Heinrich von Eckardt, the German ambassador to Mexico. The decryption of the Zimmermann Telegram is widely described as the most significant intelligence triumph for Britain during World War I. The story demonstrates that SIGINT can influence the course of History. Follow us on Twitter: @Intel_Today


RELATED POST: Key figures in UK Sigint: Conel Hugh O’Donel Alexander

RELATED POST: Alan Turing – Remembering the life of a genius [23 June 1912 – 7 June 1954]

RELATED POST: The KRYPTOS Sculpture — SECTION IV: A few clues

“The ‘Zimmermann Telegram’ of 1917 demonstrates the need to balance making use of intelligence and protecting its source, which is still relevant today.”

How GCHQ’s predecessors contributed to the US entering World War I

Because the British had severed the direct undersea telegraph links between Germany and North America in the earliest days of the war, Germany was forced to route sensitive diplomatic traffic through neutral countries.

Zimmermann’s coded message was thus transmitted through the American embassy in Berlin before passing though London and finally arriving at the U.S. State Department in Washington, D.C.

The telegram reached the German embassy in Washington on January 19, and it was transmitted to the German minister in Mexico, Heinrich von Eckardt later that day.

Unbeknownst to Zimmermann, his message had been intercepted along the way and decoded by the British cryptographic office known as “Room 40”.

The decrypted message was handed over to the United States in late-February 1917.

In the message, Zimmermann proposed a military alliance between Germany and Mexico in the prior event of the United States entering World War I against Germany.

The Zimmermann Telegram as it was sent from Washington to Heinrich von Eckardt (the German ambassador to Mexico)

The decoded telegram reads 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.


Nigel de Grey’s manuscript decrypt of the telegram. ©GCHQ

US declares war against Germany

By March 1, copy of the telegram was splashed on the front pages of US newspapers.

Diplomatic relations between Germany and the United States had already been severed in early February, when Germany resumed unrestricted submarine warfare and began preying on U.S. vessels in the Atlantic.

While many Americans remained committed to isolationism—President Woodrow Wilson had only just won re-election using the slogan, “He kept us out of war”—the Zimmerman cipher now served as fresh evidence of German aggression.

Coupled with the submarine attacks, it finally turned the U.S. government in favor of entering the fray.

On April 2, 1917, President Wilson abandoned his policy of neutrality and asked Congress to declare war against Germany and the Central Powers.

The United States would cast its lot with the Allies four days later.

“Most importantly, the Germans never realized that their diplomatic codes were readable, or, more generally, that the UK had been involved at all.”

GCHQ Official Website

WWI: The Zimmermann Telegram — BBC

As part of a BBC series looking at stories beyond the trenches, BBC Mundo’s Luis Fajardo examines the lasting impact of the Zimmermann Telegram.

The Zimmermann telegram — Decryption

The Zimmermann Telegram

Former SPY historian Thomas Boghardt returns to talk about his remarkable new account of the Zimmerman Telegram.

He has tapped fresh sources to provide the definitive account of the origins and impact of this German scheme.

Boghardt also corrects longstanding misunderstandings about how the telegram was sent and enciphered and provides a new account of how British intelligence was able to decipher it.


The Zimmermann Telegram — Wikipedia

The Zimmermann Telegram — Prologue Magazine

Real World Impact: How GCHQ’s predecessors contributed to the US entering World War I — GCHQ Website


On This Day — The Zimmermann Telegram Is Intercepted  (January 16 1917)

This entry was posted in Cryptography, SIGINT and tagged , , . Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s