Zimmermann Telegram & U.S. Entry into World War I

In January 1917 German foreign secretary Arthur Zimmermann sent a coded message proposing that Mexico ally with Germany if the United States entered the war. British codebreakers in Room 40 intercepted and deciphered the telegram, but using the intelligence created a problem: London wanted to influence American opinion without revealing how completely German diplomatic communications had been penetrated.

The British eventually passed the message to the United States through a carefully constructed cover story, and its publication in March caused widespread anger. Germany’s resumption of unrestricted submarine warfare was the more direct strategic pressure, but the telegram reinforced perceptions of German hostility. The United States declared war on Germany on 6 April 1917.

The episode brought together Room 40, wartime diplomacy, propaganda and the transition of the United States from neutrality to belligerency. Alan Bardos’s Enemies and Allies places Naval Intelligence chief Reginald Hall inside the dilemma over whether exploiting the telegram was worth risking Britain’s codebreaking advantage.