On January 17, 1917, British code breakers in Room 40, the cryptoanalysis office of Great Britain’s Naval Intelligence, intercepted a telegram from Germany. At first, they suspected the coded message was a routine communication. But, soon enough, the cryptologists found that what they held in their hands was a top-secret missive that would shift the tides of World War I.

Chances are that you have studied the Zimmermann Telegram in a history class, but have you ever actually seen the coded message? German Foreign Minister Arthur Zimmermann sent the diplomatic message to Heinrich von Eckardt, the German ambassador in Mexico City, instructing him to speak to the president of Mexico. He proposed that the two nations strike an alliance; if Mexico waged war against the United States, thereby distracting Americans from the conflict in Europe, Germany would lend support and help Mexico reclaim Texas, New Mexico and Arizona.

Leaked to the public by President Woodrow Wilson, the inflammatory contents of the message pushed the United States into the war. “No other single cryptoanalysis has had such enormous consequences,” says David Kahn, author of The Codebreakers, a seminal work on cryptology.

In its collection, the National Archives holds the coded Zimmermann Telegram, as received by von Eckardt, as well as the English translation of the telegram. Click on the yellow tabs on the documents, below, to follow the story of how the message was deciphered.

The notes are based on a conversation with Kahn and information conveyed in Barbara W. Tuchman’s book The Zimmermann Telegram and at the National Cryptologic Museum in Fort Meade, Maryland.