Imperial Germany knew that Unrestricted Submarine Warfare would eventually bring the United States into World War One on the side of the Allies. Their economic ties to Britain and public outcry over the sinking of the Lusitania and the Housatonic virtually guaranteed it. But it was hoped that it wouldn’t immediately bring America into war, and that Britain would be well on her way to starving into surrender before the Americans turned the tide in Europe.
The Founding Fathers’ caution of overseas, particularly European, entanglements, had done America well in the 19th Century. Many Americans didn’t feel Europe was worth spending American blood and treasure on, and that what happened in Europe did not affect America. Moreover, the German and Irish American immigrant communities, the two largest in the country at the time, were vehemently anti-British. In late February 1917, America was making threatening gestures and had broken off formal diplomatic relations with Germany over Unrestricted Submarine Warfare but the population wasn’t quite ready to declare war yet.
Britain knew that America had to enter the war soon, or the Allies would lose: both Russia and France were having serious internal issues in the winter of 1916/17 and Britain could not fight Germany alone. To make matters even more frustrating, Britain was sitting on a key piece of information that was sure to infuriate America against Germany. But they couldn’t release it without embarrassing themselves, and more importantly, revealing that they were reading America’s mail.
Britain had cut Germany’s transatlantic telegraph cables at the outbreak of war, but Germany asked to use America’s for diplomatic traffic. President Woodrow Wilson agreed, if only to keep a line of communication opened for peace negotiations. The Germans would deliver their messages to America’s embassy in Denmark where it would be transmitted via stations controlled by America and neutral Sweden to Washington DC. To make the long jump across the Atlantic, the message had to go through a signal boosting station at the westernmost point on the British Isles at Land’s End. Unknown to the Americans, every transmission that went through that station Britain was reading.
On 19 January 1917, the German Foreign Minister, Arthur Zimmerman, sent a coded telegram to the German embassy in Mexico via America’s diplomatic cables, which the British intercepted. The telegram was an offer for Mexico to ally itself with Germany, for which it would receive financial and economic compensation, and Arizona, New Mexico, and Texas would be ceded to Mexico after the war. (Mexico thought about it for a week. However, war would severely strain relations with Argentina, Brazil and Chile who needed a peaceful Mexico to trade with the US. Mexico, fractured by its own civil war, couldn’t defeat America if it wanted to, and even if it could, would never be able to occupy a large swath of territory populated by a people that were better armed than the Mexican Army. They politely declined.) Zimmerman’s Telegram was sure to sway American public opinion against Germany, if it could be released, and once released, believed to be genuine.
British agents investigated the route that the message would have traveled from Washington DC to Mexico City, and found that it had not gone directly to the German embassy, but to a Mexican telegraph office down the street. The British concocted a story that an agent “Mr. H” acquired a copy from that office. This was enough cover for their operation at Land’s End (which would continue for another 25 years), and furthermore, force the Germans to suspect a spy in their embassy. Convincing the Americans it was genuine would be trickier: to do that they would have to acknowledge they broke Germany’s code. Fortunately, Germany came to the rescue, and changed their codes on 1 February with the beginning of Unrestricted Submarine Warfare. Britain was now free to release the telegram, the cover story, and the older code for verification to America.
On 19 February 1917, the British released the Zimmerman Telegram to the American embassy in London, and on the 25th it was given to President Wilson. Wilson was furious, and on the 28th leaked it to the press.
The American people were predictably outraged, but anti British sentiment called it a forgery, or pushed that outrage against Mexico. Blackjack Pershing was already chasing Pancho Villa, and the Carranza government in Mexico didn’t want any more American troops on its soil. On 3 March, in one of those ironic and unintended consequences on which history seems to turn on occasion, Zimmerman was forced to verify the authenticity of the Telegram in order to maintain Mexico’s neutrality with Germany. This all but silenced the anti-British sentiment in America. It was one thing to avoid getting entangled in Europe’s affairs, but quite another when Europe tries to entangle itself in American affairs first. Germany “needed to be punished”.
It wouldn’t be long before Wilson would break his campaign promise to keep America out of the war.