From the Facebook page of The Great War:
On this day 100 years ago, a coded telegram was sent by German foreign secretary Arthur Zimmermann to German Ambassador to Mexico, Heinrich von Eckardt. In this telegram, Zimmermann instructed von Eckardt to offer Mexico a military alliance and financial support against the United States should they not remain neutral. This was a possibility since Germany was about to unleash unrestricted submarine warfare by February 1, 1917.
To understand this telegram, it is important to understand that talks about military cooperation and even a military alliance between Mexico and the German Empire had been going on since 1915 already.
The telegram was sent via the American undersea cable since the German cable was interrupted by the British when the war broke out. US President Woodrow Wilson had offered the Germans to use their cable for diplomatic correspondence. What neither Wilson nor the Germans knew: The cable was monitored by the British intelligence at a relay station in England. Furthermore, the British codebreakers of Room 40 had already cracked the German encryption.
The biggest challenge for the British now was to reveal the content of this telegram without admitting that they were monitoring the cable while ensuring it had the desired impact.