In January, 1917, British intelligence intercepted a telegram from the German foreign ministry to its ambassador in Mexico. The message, which has come to be known as the “Zimmermann Telegram,” detailed a proposal by the German government to support a Mexican attack on the United States if the U.S. were to declare war on Germany.
Mexico was still smarting from U.S. incursions into its northern borderlands by the United States Army, as well as the seizure of Veracruz in 1914. German leadership hoped that war with Mexico would delay or reduce the amount of assistance the United States could offer the European allies. This would improve Germany’s chances of success in 1917 and 1918.
British codebreakers had a conundrum, though. How to get the telegram into the hands of the Americans without giving away the fact that they were tapping American diplomatic channels? The Americans, officially neutral in the war and hoping that a negotiated peace could be brokered, allowed German diplomatic traffic to pass over their trans-Atlantic cables. Normally, this traffic had to be unencrypted, but somehow Germany was able to convince American diplomats to allow this telegram to be sent encoded. Since the cable ran through British hands, and our cousins across the sea are nobody’s fools, they were making copies of everything that went down that wire.
After a bit of subterfuge on the part of the British, and a bit of bad decision-making on the part of the Germans, the telegram was not only delivered to the Americans, but was publicly confirmed as authentic . This helped to swell anti-German sentiment in the United States and, along with German resumption of unlimited submarine warfare in February 1917, helped to bring the Americans into the war against Germany.