The Zimmermann telegram was a message from German Foreign Secretary Arthur Zimmermann, proposing an alliance between his country, Mexico and Japan against the United States in World War I.
It was intercepted and decrypted by the British, and its publication provoked an outrage and pushed the heretofore-reluctant United States into the war.
Its decrypting is held as the cryptological feat that has most influenced History.
Context
In early 1917 Germany, blockaded by the British Royal Navy, was starved for resources. It felt it had to employ unrestricted submarine warfare (the indiscriminate sinking by U-boats of ships—even unarmed and from neutral nations—found in the war zone) to defeat the British. This, however, would risk the United States, still neutral despite the loss of American lives due to the occasional sinking of ships by the Germans, entering the war on the Entente’s side.
Germany devised a plan to keep the United States otherwise occupied: it would propose Mexico an alliance against the United States, and Mexico could regain territories it lost after the Spanish-American war of 1846-1848.
This proposal was sent but the telegram was intercepted and decoded by the British and given to the Americans.
Influence
The publication of the telegram united the American public in favor of entering the war against Germany, and left no choice to President Wilson, who had been reluctant and had just been re-elected with campaign slogan “he kept us out of war”.
It is probable that the United States would have joined the war even if the Zimmermann telegram had not been intercepted, especially with Germany’s resuming unrestricted submarine warfare, but the telegram precipitated the event and unified the country behind the effort.