In January 1917 the German diplomatic representative in Mexico received a secret telegram penned by German Foreign Secretary Arthur Zimmermann.
It proposed forming a secret alliance with Mexico if the United States should enter the war. In return, if the Central Powers were to win the war, Mexico would be free to annex territory in New Mexico, Texas and Arizona.
Unfortunately for Germany, the telegram was intercepted by the British and decrypted by Room 40.
On discovering its contents the British hesitated at first in passing it on to the Americans. Room 40 didn’t want Germany to realise they had cracked their codes. And they were equally nervous about America discovering they were reading their cables!
A cover story was needed.
They guessed correctly that the telegram, having arrived first in Washington by diplomatic lines, would then be sent on to Mexico via commercial telegraph. A British agent in Mexico was able to retrieve a copy of the telegram from the telegraph office there – that would satisfy the Americans.
To cover up their cryptographic activities, Britain claimed to have stolen a decrypted copy of the telegram in Mexico. Germany, unwilling as ever to accept the possibility that their codes might be compromised, swallowed the story completely and began turning Mexico City upside-down looking for a traitor.
Germany’s reintroduction of Unrestricted Submarine Warfare in early January 1917, putting American shipping in the Atlantic at risk, led America to sever diplomatic ties on 3rd February. This new act of aggression was enough to make war inevitable.
President Woodrow Wilson granted permission for the telegram to be made public and on 1st March the American public awoke to find the story splashed across their newspapers.
Wilson won his second term of office in 1916 with the slogan “he kept us out of the war”. But keeping to that course had become ever more difficult in the face of increasing German aggression. Now pubic opinion had turned.
On 2nd April President Wilson asked Congress to declare war on Germany and the Central Powers.
The letter from United States Ambassador to the United Kingdom Walter Hines Page to American Secretary of State Robert Lansing:
Title image: the encrypted Zimmermann Telegram.