The Secret Message Became Front-Page News

1. A coded proposal

In January 1917, German foreign secretary Arthur Zimmermann sent a coded message proposing an alliance with Mexico if the United States entered the war. The message concerned a possible alliance with Mexico. Its importance depended on both the proposal and the international situation around it. A secret offer could have consequences beyond its intended reader.

The proposal concerned territory belonging to its intended opponent.

2. A proposal meant for a private reader

The telegram was not written as a public declaration for Americans to debate. It was a secret diplomatic proposal concerning Mexico and a possible future conflict. Its importance changed once people outside the intended exchange could read it, because the proposed arrangement brought the European war into a different country's security concerns.

The unintended audience was the one that changed the story.

3. The offer

Germany suggested that Mexico could regain Texas, New Mexico and Arizona. The proposal was conditional, not an agreement that Mexico actually carried out. Intercepting a message was only the first difficulty. The codebreakers still needed to establish what its unfamiliar groups of characters meant.

A secret offer is not the same thing as an accepted invitation.

4. A British interception

British cryptographers deciphered the telegram. Revealing its contents presented a second problem: protecting the intelligence methods that had exposed it. Publishing intelligence can expose how it was acquired. The British also had to consider how to share the discovery without compromising their sources.

Knowing the secret created another secret to protect.

5. The public reveal

Britain shared the information with President Woodrow Wilson in February. Publication in March intensified American anger toward Germany. The proposal gave the American public a specific document to consider. Its contents connected a distant war with concerns closer to home.

A private diplomatic message had acquired millions of readers.

6. Protect the discovery too

British officials faced two connected problems: how to use the information, and how to avoid revealing too much about the intelligence work behind it. Sharing a secret can expose the way it was obtained. The message therefore had to travel through political decisions as well as the original communications network.

The evidence came with a secret of its own.

7. Part of a turning point

The United States declared war in April 1917. The telegram influenced opinion alongside other developments, including unrestricted submarine warfare. American entry into the war had more than one cause. The telegram mattered without being treated as the only event behind the decision.

The code failed, and its audience changed history.

Sources

• U.S. National Archives — Zimmermann telegram