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But in January 1917, British code breakers intercepted an encrypted message from German foreign secretary Arthur Zimmerman intended for the German ambassador to Mexico that would change the

course of history. After weeks of decoding, the cryptographic team discovered a now-famous set of instructions - revealing that Mexico would be offered a secret wartime alliance if the US

chose to go to war on the side of the Allies. According to historian Mr Gregory, Zimmerman’s message was simple: “Make war together, make peace together.” If it did, Mexico could expect huge

financial payouts in exchange for attacks on “lost territory in the United States, in Texas, New Mexico and Arizona.” The communication was handed over to US officials by late February, and

by March had been splashed over the front pages of newspapers nationwide in a massive blow to US-German relations. Public opinion was “inflamed”, and within just months President Wilson

abandoned his policy of neutrality - asking Congress to declare war against Germany. Mr Gregory said: “Armed neutrality had been ineffectual. It had proved impracticable in defending

shipping against attack from "outlaw" German submarines.” “Two days later the Senate voted in favour of Wilson's resolution, and two after that it was endorsed by the House of

Representatives.  “Later that same day, 6 April 1917, the President signed his official declaration.  "America was at war.”