The Zimmermann Telegram

British codebreakers intercepted and decoded a German message proposing an alliance with Mexico. When it was published, it helped turn American opinion toward joining World War I.

Context

German Foreign Secretary Arthur Zimmermann sent the message in January 1917 to the German minister in Mexico, Heinrich von Eckardt. It said that if the United States entered the war after Germany resumed unrestricted submarine warfare, Germany would propose an alliance with Mexico. In return Mexico would receive support in recovering territory lost to the United States, which the telegram named as Texas, New Mexico and Arizona.

British naval codebreakers in the Admiralty's Room 40 deciphered it in January 1917. Britain did not want to reveal that it was reading German traffic, so it first obtained a copy of the version sent from Washington to Mexico City and gave the text to the US government on 24 February.

The American press published the story on 1 March 1917. Zimmermann then publicly confirmed that he had sent it, which undercut claims that it was a forgery. Congress declared war on Germany on 6 April 1917. Historians generally treat the telegram as one factor among several, alongside the submarine campaign, and Mexico never took up the offer. Zimmermann resigned as foreign secretary later in 1917. The affair also showed how signals intelligence could shape diplomacy, since the decisive evidence came from reading another government's private traffic.