Zimmermann telegram

The Zimmermann telegram was a secret diplomatic communication that altered the course of World War I. Issued by the German Foreign Office on January 17, 1917, it proposed a military alliance between the German Empire and Mexico. This message was a piece of signals intelligence, which refers to information gathered from intercepted communications. Its discovery and revelation became one of the most significant intelligence triumphs for Great Britain. The telegram's contents helped drive the United States to declare war on Germany in April 1917.

The message was dispatched by Arthur Zimmermann, a high-ranking civil servant in the German Foreign Office. He sent the coded telegram to Heinrich von Eckardt, the German ambassador in Mexico. Zimmermann sent this note in anticipation of Germany resuming unrestricted submarine warfare on February 1, 1917. Unrestricted submarine warfare meant German U-boats would attack all ships, including neutral ones. The telegram instructed Von Eckardt to propose a military alliance to the Mexican government if the United States entered the war.

Germany had several strategic motivations for this proposal. The primary goal was to prevent the United States from sending troops and war materials to the Allies. By inciting a conflict between Mexico and the United States, Germany hoped to tie down American forces on their own continent. This would slow the export of American arms to the European battlefields. The German High Command believed they could win the war on the Western Front before American forces arrived in large numbers. They intended to use submarine warfare to strangle British supply lines in the process.

The content of the telegram included a specific promise of land to Mexico. Germany suggested that, with German aid, Mexico could recover territories lost to the United States. These territories included Texas, New Mexico, and Arizona.

This plan was part of a longer German effort to influence Mexico. In 1915, a German officer named Franz von Rintelen had even attempted to provide $12 million to Mexico to incite war. However, the 1917 proposal was much more direct and formal in its diplomatic approach.

British intelligence successfully intercepted and decrypted the message in a specialized unit called Room 40. This group of codebreakers worked at the Admiralty. They were able to crack the German diplomatic code, known as 13040, using various methods. One breakthrough occurred after the failure of the Niedermayer–Hentig Expedition to Afghanistan. During that mission, a man named Wilhelm Wassmuss abandoned a codebook that the Allies later recovered. This helped the British understand German communications.

Despite the successful decryption, British leaders were cautious about revealing their success. William Reginald Hall, the chief of Room 40, hesitated to share the news immediately. He feared that telling the Americans would reveal that Britain had broken German codes. He also worried about exposing British eavesdropping on United States diplomatic traffic. After several weeks, cryptographers Nigel de Grey and William Montgomery completed the decryption. The truth was eventually revealed, and the news enraged the American public. Arthur Zimmermann publicly admitted the telegram was genuine on March 3, 1917.

Mexican President Venustiano Carranza responded by assigning a military commission to study the German proposal. The commission concluded that the war was unwinnable for Mexico. They noted that Mexico was already struggling with an internal civil war. Furthermore, the United States possessed a much stronger military than Mexico. The commission also found Germany's promises of "generous financial support" to be unreliable. Germany had previously told Carranza they could not provide enough gold for a Mexican national bank. Consequently, Mexico remained neutral during World War I.