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Zimmermann Telegram (broken by Room 40)
sent January 16, 1917; made public March 1, 1917· Book, artwork, monument or document· Technology, codes & intelligence
Well documentedSolid records, physical evidence or scientific consensus.
A secret German message proposing a war alliance with Mexico, decoded by British codebreakers and published in March 1917.
See it on the Battles & Wars map →What happened
On January 16, 1917, German Foreign Secretary Arthur Zimmermann cabled his minister in Mexico: Germany would resume unrestricted submarine warfare on February 1; if the United States entered the war, Mexico should join Germany, 'reconquer' Texas, New Mexico and Arizona, and invite Japan in. Britain's Room 40 codebreakers read it. To hide their source, they obtained the copy sent on to Mexico, then passed it to the Americans. It was published on March 1, and Zimmermann admitted it was genuine.
Why it matters
It turned American opinion, especially in the West and Southwest, toward war, and showed how codebreaking could change a great power's policy.
How we know
The intercept and decrypt in British archives, US State Department records, newspaper coverage, and Zimmermann's own public admissions on March 3 and 29, 1917.
Still debated
Why Zimmermann admitted it is still puzzling: many Americans first suspected a British forgery, and a denial might have kept that doubt alive.
Connections (3)
▸Electric telegraph and Morse code carried, by undersea cable, the Zimmermann Telegram (broken by Room 40)Well documented
The connection
In August 1914 Britain cut Germany's undersea cables, so Germany had to send overseas messages through other countries' lines. The Zimmermann message went by the American diplomatic cable, which President Wilson had let Germany use for peace talks, and by a Swedish route. Both passed through Britain, where Room 40 copied and decrypted it.
When and how
English Channel, August 1914 (cable ship Alert); Berlin and London, January 16–17, 1917; Room 40 codebreakers Nigel de Grey and William Montgomery, under Admiral William Hall.
How we know
British Admiralty files and the decrypt itself, now in the UK National Archives, and US State Department records of the diplomatic cable.
Debates and open questions
Which route Room 40 first read the message from was long kept secret, and historians still disagree on the details.
▸Zimmermann Telegram (broken by Room 40) helped bring the United States into World War IWell documented
The connection
Published on March 1, 1917, the telegram outraged Americans, especially in the West and Southwest, where a Mexican threat felt real. Zimmermann confirmed it two days later. With U-boats sinking American ships that March, President Wilson asked Congress for war on April 2, and the United States declared war on Germany on April 6, 1917.
When and how
Washington, February 24 – April 6, 1917; Wilson, US Ambassador Walter Hines Page in London, and Britain's Admiral William Hall.
How we know
Newspapers of March 1917, Wilson's war message, which cites the telegram, and the congressional record.
Debates and open questions
Most historians think the U-boat campaign was the main cause; the telegram's weight lay in swinging public opinion.
▸Zimmermann Telegram (broken by Room 40) accompanied Germany's return to U-boats and unrestricted submarine warfareWell documented
The connection
The telegram's first line announced Germany's plan: from February 1, 1917, U-boats would sink any ship, neutral or not, in the waters around Britain. Germany's leaders expected this might bring in the United States, so they offered Mexico an alliance to keep Americans busy at home. They gambled that U-boats would starve Britain before American troops arrived.
When and how
Pless Castle, Silesia, January 9, 1917, where Kaiser Wilhelm II approved the campaign; Zimmermann's telegram, January 16, 1917.
How we know
The telegram's text and German records of the Pless decision, including the navy's memo predicting Britain's collapse within months.
Debates and open questions
Whether German leaders really expected Mexico or Japan to act is doubtful; Mexico's president, Venustiano Carranza, studied the idea and declined.