Zimmermann telegram: secret proposal to public admission
The Zimmermann telegram was a German diplomatic message proposing an alliance with Mexico if the United States entered World War I against Germany. British intelligence intercepted and decoded it, but making its contents public created another problem: how could Britain establish its authenticity without revealing its intelligence methods? Many Americans initially regarded the message as a British forgery. Arthur Zimmermann, Germany's State Secretary for Foreign Affairs, then publicly confirmed that it was genuine.
Zimmermann directed the message to Heinrich von Eckardt, the German ambassador to Mexico. Germany intended to resume unrestricted submarine warfare on February 1 and anticipated that this would almost certainly lead to war with the United States. If American participation became certain, the ambassador was to approach Mexico with an offer of German financial backing for a military alliance. Mexico would recover Texas, Arizona, and New Mexico, with the details left for the ambassador to arrange. The proposal also asked Mexico's president to invite Japan to join and to help mediate between Germany and Japan. Secrecy was part of the instructions.
The strategic purpose of the Zimmermann telegram was to occupy American forces closer to home and slow American arms exports to the Allies. German leaders hoped to defeat Britain and France before American troops could arrive in Europe in sufficient strength. Mexico's president, Venustiano Carranza, asked a military commission to examine the proposal. Mexico was already in a civil war, Carranza's position was insecure, and the United States possessed greater military strength. An attack could also prompt Washington to support one of Carranza's rivals, adding another threat to his government.
Money and diplomacy created further obstacles for Mexico. German financial promises were unreliable: Germany had already told Carranza it could not provide the gold required for an independent Mexican national bank. Even with funding, Mexico would have needed to purchase weapons and ammunition from Argentina, Brazil, and Chile, risking strained relations. Those countries had helped organize the Niagara Falls peace conference after the American occupation of Veracruz. Recovering the proposed territories would also require governing a large English-speaking population accustomed to self-government and comparatively well armed. Meanwhile, Japan was already fighting alongside the Allies and later declared no interest in changing sides or attacking America.
The route of the Zimmermann telegram made its interception particularly sensitive. Britain had cut Germany's international telegraph cables when the war began. Germany therefore used other routes, including diplomatic traffic handled for neutral countries. President Woodrow Wilson had permitted German use of American diplomatic communications to encourage relations and efforts toward a negotiated peace. Although the Americans initially required uncoded messages, Wilson later relaxed that condition. Zimmermann's coded message was accepted for transmission through the American channel. Traffic passing through British hands reached the Admiralty's Room 40, where codebreakers and analysts were working on German communications. Nigel de Grey partially decoded the message by the following day.
Room 40 chief William Reginald Hall faced competing concerns. Publication might influence American opinion, but it could reveal both British access to German codes and interception of American diplomatic traffic. He waited while de Grey and William Montgomery completed the decryption. The British then obtained a copy of the coded message from a commercial telegraph office in Mexico through an agent who bribed an employee. That retransmitted version used an older diplomatic code, 13040. It offered a way to disclose the contents while concealing the extent of British access to newer codes. American commercial telegraph files also provided a route for independent verification by the American government.
In February, the Zimmermann telegram passed through American diplomatic hands in Britain. Edward Bell initially doubted it, then accepted its authenticity. Ambassador Walter Hines Page received the coded message, German wording, and English translation from British Foreign Minister Arthur Balfour. Page reported to Wilson, including details that could be checked against American telegraph-company files. Wilson wanted publication but delayed until March 1. Public suspicion persisted because the explanation that Britain had obtained an already decoded message in Mexico concealed the true intelligence operation. Some antiwar newspapers, along with German and Mexican diplomats, promoted the forgery interpretation. Wilson could not freely publish the confidential evidence without compromising British codebreaking.
Zimmermann's own statements removed that obstacle. At his March 3 press conference, he told an American journalist, "I cannot deny it. It is true." On March 29, he acknowledged the Zimmermann telegram again in the Reichstag, emphasizing that German support for Mexico depended on the United States joining the war. Publication helped build American support for a declaration of war, alongside anger over unrestricted submarine warfare. Congress declared war on Germany on April 6, 1917.