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By the beginning of 1917, the First World War had reached a stalemate. The Allies hoped that America would enter the war and break the deadlock, but US President Woodrow Wilson was steadfastly refusing to send in American troops. As well as not wanting to sacrifice his nation's youth in the bloody battlefields of Europe, Wilson was convinced that the war could only be ended by a negotiated settlement, and he believed that he could best serve the world if he remained neutral and acted as a mediator.

Then, on 16 January 1917, British Intelligence intercepted a telegram sent from German Foreign Minister Arthur Zimmermann to the German ambassador in Washington. The telegram contained details of a massive German naval offensive and a plan to keep America out of the war, but this was not immediately apparent to the British, because the telegram had undergone routine encryption before being transmitted. It was sent to the naval codebreaking bureau in Whitehall, otherwise known as Room 40, where it was handed to Nigel de Grey, one of Room 40's foremost codebreakers. If he could crack the German code, he would provide the Allies with an invaluable warning of what was about to happen. If he failed, the Kaiser would be able to implement his plan without hindrance. The course of the war lay in de Grey's hands.

The story of the Zimmermann telegram is undoubtedly the single most important episode in the history of wartime cryptography. It demonstrates the value of secure communication against the catastrophe of a broken code.