In an effort to tip the balance in their favor, and still keep the Americans out of World War I, Germany devised a plan and asked Mexico for its assistance. The plan involved cutting off supply lines to Britain and France by beginning unrestricted submarine warfare in the Atlantic. Fearing the United States would join the battle if their ships were sunk, Germany asked Mexico to start a war with the United States and promised the return of Texas, Arizona, and New Mexico.
Germany hoped a conflict in the Southwest would keep the Americans out of the European war. The request was sent from the foreign minister in Berlin, Arthur Zimmermann, through the German ambassador in Washington DC to the German ambassador in Mexico City, in the form of a coded message. It became known as “The Zimmermann Telegram” and it changed the course of world history.
Britain intercepted the message as it was transmitted overseas. Royal Navy cryptanalysts decoded it. Realizing they held the key to the United States’ participation in the war, Britain, through a little subterfuge, eventually showed the message to the United States. There were those in the Congress that suspected Britain of creating a false message so that the United States would lend assistance to their fight. To verify its authenticity, a copy of the Western Union message that was sent from DC to Mexico City was supplied and Britain’s cryptanalysts decoded it in the presence of the U.S. ambassadors to England. This convinced the doubters and Congress declared war on Germany. Thus, a single coded message, and the efforts of cryptanalysts, changed history.