Zimmermann Telegram, as sent from Washington to Mexico City. Wikipedia image.

Today marks the 100th anniversary of the Zimmermann Telegram, which played a key role in the entry of the United States into the First World War.  The message was originally sent from Germany on January 11, and on January 19, 1917, it made the final leg of its circuitous route to Mexico City.

Interestingly, the telegram was routed through Britain, which allowed the British to intercept and decode it.  Because the Germans had no wire communications with America, President Wilson allowed them to send diplomatic cables courtesy of the American embassy in Copenhagen.  That was routed through Britain, where the British were able to intercept it.

From the German embassy in Washington, the cable was sent to Mexico City by Western Union.  The telegram instructed the German ambassador to Mexico to propose, if hostilities appeared imminent, an alliance between Germany and Mexico.