The intercepted telegram that helped pull America into World War I

The story

In January 1917, Arthur Zimmermann, Germany’s foreign secretary, sent a secret message to the German ambassador in Mexico. If the United States entered the war, Germany proposed an alliance with Mexico and promised help to win back Texas, New Mexico and Arizona.

Britain had cut Germany’s undersea telegraph cables at the start of the war. So the message was sent in code over other routes, including American diplomatic cables that passed through British hands. Germany assumed the code kept it safe.

British codebreakers in a unit known as Room 40 had been working on that code for years. They had partly decoded the telegram by the next day.

Britain then had a problem: revealing the message would show it had been reading American diplomatic traffic. It handed the text to the United States with a cover story. Zimmermann himself confirmed in March that the telegram was real, and the United States declared war on Germany in April 1917.

Why it mattered

The decryption has been described as Britain’s most significant intelligence success of the First World War, and it helped build support for the American declaration of war in April 1917. It was one of the earliest times a decoded message changed world events. Germany trusted its code and never questioned whose lines its message was traveling over.

The lesson today

Germany relied on a code it had used for years and on cables it did not control. Old encryption weakens over time, and messages that pass through other people’s systems can be copied along the way. Keeping encryption current, and knowing who handles your data, still matters.