British signals intelligence intercepted and decrypted a coded German telegram from German Foreign Secretary Arthur Zimmermann that was intended for Germany’s ambassador to Mexico. Though the United States was not yet a participant in the war, Zimmermann signaled Germany’s intention to unleash its U-boats in the Atlantic Ocean in a fully unrestricted campaign against all Allied shipping, including American transports.
The telegram further conveyed Germany’s intentions should America enter the war. That included urging Mexico to join Germany in declaring war against the United States. In exchange, Germany committed to assist Mexico in regaining its lost territories of Arizona, Texas, and New Mexico. A second intercepted telegram from Zimmermann instructed the German Embassy not to wait until the United States joined the war to present its offer to Mexico.
The British shared the original message with U.S. officials on February 24. Though some American officials suspected the telegram to be a British ruse intended to induce a declaration of war, President Woodrow Wilson saw it as genuine. When he first learned of Germany’s duplicitous maneuvering, the President cried out “Good Lord! Good Lord!” in outrage. Wilson was further galled that the message was transmitted using an American undersea telegraph service he had offered to the Germans as a courtesy at the outset of hostilities in 1914.
While Wilson shared a copy of the telegram with Capitol Hill, he also instructed Secretary of State Robert Lansing to leak the contents to the Associated Press. On March 1, 1917, the story fanned across America, stirring considerable public anger against Germany. Two days later, Foreign Secretary Zimmermann inexplicably confirmed the contents of the telegram at a press conference in Berlin, putting to rest any doubts about its authenticity and hardening U.S. public opinion against Germany.
On April 2, President Wilson convened a joint session of Congress and called for a declaration of war. It came just four days later. One of America’s first actions in the war was to take control of the more than 90 German and Central Powers ships that were interned in U.S. harbors and convert many of them into transport vessels for use by the U.S. Navy. German efforts to keep the United States out of the war had backfired, and the American entry into the war would mark a turning point in favor of the Allied Powers.