On 17 January 1917, in a cramped office inside the British Admiralty known as Room 40, codebreakers Nigel de Grey and William Montgomery began teasing meaning out of a freshly intercepted German telegram. Even the fragments they could read were extraordinary. The message came from Arthur Zimmermann, Germany's foreign secretary, and was addressed to the German minister in Mexico City. It proposed something that sounded like fiction: if the United States entered the war against Germany, Mexico should be invited to join the German side — and reconquer its "lost territory in Texas, New Mexico and Arizona."
The context made the message urgent. Germany had decided to resume unrestricted submarine warfare on 1 February 1917, sinking without warning any ship, neutral or not, in the waters around Britain. Berlin knew this would probably provoke America. The telegram was the hedge: if Washington did declare war, a German-financed Mexican front would keep American troops busy at home. The instructions to the German minister, Heinrich von Eckardt, also suggested Mexico broker Japanese entry into the alliance.
The delicious irony is how the telegram travelled. Britain had cut Germany's transatlantic cables at the start of the war, so Berlin sent sensitive traffic by routes it believed safe — including, astonishingly, the American diplomatic cable service, which President Woodrow Wilson had allowed Germany to use in the hope of facilitating peace negotiations. Zimmermann's message went from Berlin to the US embassy, on to Washington, and then down to Mexico City by commercial telegraph. British intelligence was quietly reading the traffic all along.
Blinkers off, sources hidden
That created an exquisite dilemma for Room 40's chief, Admiral William "Blinker" Hall. Handing the telegram to the Americans would reveal two secrets Britain badly wanted to keep: that it could break German diplomatic codes, and that it was intercepting messages carried on American cables. Hall waited weeks while a solution was engineered. A British agent obtained a copy of the telegram as it had been retransmitted from Washington to Mexico City — a version encoded in an older cipher, 13040, and bearing different dates and markings. Britain could now plausibly claim the message had been acquired in Mexico, protecting both the codebreaking operation and the awkward matter of the tapped cables.
In late February 1917 the text was handed to Walter Hines Page, the American ambassador in London, who cabled it to Wilson. The president, already furious about the submarine campaign, authorised its release, and on 1 March the telegram exploded across American front pages. Many Americans — including some sceptical newspapers — initially suspected a British forgery designed to drag the United States into the war. Then Zimmermann did the propagandists' work for them. At a press conference on 3 March, instead of denying the message, he confirmed it was genuine, and he repeated the admission in a Reichstag speech later that month. He argued the proposal was purely conditional — it would only apply if America entered the war — but the distinction was lost on a public reading about a plot to carve up the American Southwest.
From telegram to declaration
Mexico, for its part, never seriously bit. President Venustiano Carranza had a commission study the offer, which concluded the obvious: Mexico could not hope to defeat the United States or hold Anglo-populated territory, and German gold and guns could hardly reach Mexico through the British blockade anyway. The proposal was formally declined.
But the damage to German–American relations was done. Combined with the mounting toll of unrestricted submarine warfare on American shipping, the telegram collapsed what remained of US neutrality — a neutrality Wilson had campaigned on only months earlier under the slogan "He kept us out of war." On 2 April 1917, Wilson asked Congress for a declaration of war, and on 6 April the United States formally entered the First World War against Germany. The telegram was not the sole cause, but it transformed the argument: this was no longer a distant European quarrel, it was a plot aimed at American soil.
The episode is also a landmark in the history of intelligence. It is often cited as the most consequential single act of signals intelligence ever: a war's outcome tilted by cryptanalysis. Room 40's veterans and methods flowed after the war into the Government Code and Cypher School — the organisation that would move to Bletchley Park in 1939 and break Enigma. And the lesson Hall taught endured: breaking a code is only half the job; the other half is using the secret without revealing that you have it.
Quiz nuggets
- Arthur Zimmermann, Germany's foreign secretary, proposed a German–Mexican alliance in January 1917, offering Mexico Texas, New Mexico and Arizona.
- The telegram was decoded by Room 40, the British Admiralty's codebreaking unit.
- Germany sent the message partly via American diplomatic cables that Wilson had made available for peace talks.
- Zimmermann himself confirmed the telegram was genuine at a press conference on 3 March 1917.
- The United States declared war on Germany on 6 April 1917, weeks after the telegram was published.