The Zimmerman Telegram

A secret communication between Germany and Mexico was exposed on 3 March 1917.

Zimmermann Telegram as Received by the German Ambassador to Mexico (detail)

Arthur Zimmermann, State Secretary for Foreign Affairs of the German Empire, was tasked with keeping the US out of the First World War. His plan was as ingenious as it was unlikely. He decided to offer Mexico financial and military support should it agree to attack the US and attempt to regain the territories lost in the Mexican-American War of 1846-48. This would keep the US occupied – and out of Europe.

On 19 January, Zimmermann sent a coded telegram to the Mexican president, via the German ambassadors in the US and Mexico, outlining the plan. This was his first mistake. What neither Zimmermann, nor indeed the Americans, knew was that the British were tapping the line.

The telegram was intercepted and decoded, but it left British intelligence in a tricky situation. Here was the evidence needed to get the US into the war, but if they went public the Americans would know that the British were tapping their diplomatic traffic and the Germans would know that their code had been cracked.

The British had a plan. An agent, known as ‘Mr H’, bribed a telegraph office worker in Mexico City for a copy, which he knew they would have. This was shown to the Americans who put out a cover story that it had been stolen from the German Embassy in Mexico. The German High Command would consider this far more likely than their codes being broken.

Even at this late stage Zimmermann could have salvaged his plan. The proposed Mexican-German alliance was so unlikely that many in the US simply refused to believe it. It seemed more probable that the British had invented the story to drag them into the war. On 3 March, however, in an astonishing move, just when even the US press was getting behind the forgery idea, Zimmermann confessed that the telegram was genuine. On 2 April, President Wilson asked Congress to declare war on Germany.