In late 1916 the war in Europe had been raging for more than two years, and victory did not appear to be in sight for either side.  On the Eastern Front there were signs that the Russians might be cracking, and outright revolution was increasingly likely.  If this happened, and if Russia withdrew from the war, Germany could shift its entire focus to the west.  There, the German and Allied armies had been bogged down in trench warfare since the autumn of 1914. Perhaps the transfer of German divisions to the Western Front would be sufficient to break the deadlock; but to make victory more certain some members of the German high command believed that Britain could be forced out of the war by a resumption of unrestricted submarine warfare.  The Germans had curbed their attacks on certain types of shipping following the furor over the sinking of the Lusitania in 1915.  

German leaders were aware that a resumption of unrestricted submarine attacks on Allied and neutral (mostly American) shipping around the British isles might bring the United States into the conflict.  They also knew, however, that America was almost totally unarmed and had an army of negligible size whose only war experience was in fighting Mexican bandits.  By the time the United States equipped and trained an army and sent it to Europe, they believed the war would be over. But should America enter the war, members of the German Foreign Office thought it possible and desirable to incite a war between Mexico and the United States to tie down American forces and slow the export of arms to the Allies.  The Mexicans certainly hated the United States enough, and there was considerable pro-German sentiment among the Mexican leaders.  To encourage the Mexicans to act, on January 16, 1917, German Foreign Minister Arthur Zimmerman sent a message to Mexico City proposing a military alliance against the United States, with the return of Texas, New Mexico, and Arizona among other rewards promised to Mexico for their cooperation.

Sixteen days later, on February 1, Germany resumed unrestricted submarine warfare.

German undersea cables to North America had been cut by the British, so the Germans, by previous arrangement with the American government, used the American diplomatic circuit to transmit Zimmerman’s enciphered message to the German ambassador in Washington.  The ambassador then forwarded the message to Mexico City. 

The transatlantic cable service from Europe ran through a relay station near Lands End England, and the British, unbeknown to the Americans, tapped in and obtained a copy of Zimmerman’s telegram. British cryptologists were able to decipher its contents in reasonably short order. Although they realized its explosive potential, the British sat on the information for some weeks while trying to concoct a story of how they managed to get the message and how they decrypted it.  They did not wish the United States to know that they were tapping its cables, and they did not want the Germans to know that they were exploiting one of their high-level ciphers. Finally, as a cover story, the British publicly claimed that their agents had stolen the telegram’s deciphered text in Mexico.  Privately, when they gave the American ambassador a copy of the message on February 19, they were able to give him sufficient background information to prove the telegram’s authenticity.

When news of the Zimmerman telegram was published in American newspapers, there was outrage.  Many voices were raised in a call for war. Yet there was considerable anti-British sentiment in the United States, especially within Irish and German enclaves, and the anti-war groups raised charges of forgery.  Knowing the truth, yet unable to prove the authenticity of the Zimmerman telegram without harm to future British cryptologic efforts, President Wilson remained silent.  Then, unexpectedly, Zimmerman himself came to the truthtellers rescue. First, at a press conference on 3 March 1917, he told an American journalist, “I cannot deny it. It is true.” Then, on 29 March 1917, Zimmermann gave a speech in the Reichstag in which he admitted the telegram was genuine.  Zimmermann said that he hoped Americans would understand that Germany would only fund Mexico’s war with the United States in the event of American entry into the European war.

This glaring example of German realpolitik did not favorably impress many Americans.

On April 6, 1917, Congress voted for war with Germany.

The Mexicans mulled over Zimmermann’s offer but wisely decided to remain neutral.

One final note:  I always thought that the Zimmerman Telegram would be a grand subject for a tight cinematic drama.  Also, one might note that the Zimmerman Telegram, though perhaps not as impactful,  may be compared  with the famous Ems Dispatch that ignited the Franco-Prussian War.