Germany secretly offered Mexico Texas, Arizona, and New Mexico if they'd go to war against the US — and Britain intercepted every word.
On January 17, 1917, German Foreign Secretary Arthur Zimmermann dispatched a coded telegram to Mexico proposing a secret military alliance. The deal was breathtaking: Germany would provide financial support and help Mexico reclaim Texas, New Mexico, and Arizona from the United States.
The message never reached its intended recipient in secret. British codebreakers in Room 40 — the Admiralty's secret intelligence unit — intercepted and decrypted the telegram before it even arrived in Mexico City.
Britain sat on the intelligence for weeks, carefully planning how to reveal it without exposing that they had cracked German diplomatic codes. They eventually staged a leak that made it appear the telegram had been obtained in Mexico, protecting their surveillance operation.
When the telegram's contents were published in American newspapers on March 1, 1917, the public reaction was explosive. Many Americans had been resistant to entering the European war, but the idea of Germany arming Mexico against them on their own border changed everything.
In a remarkable moment, Zimmermann himself confirmed the telegram was real at a press conference on March 3, 1917. Most diplomatic forgeries are denied — his admission stunned the world and made the crisis impossible to contain.
Mexico quietly declined the offer after their military concluded the war against the US was unwinnable. They had nothing to gain and everything to lose by taking on a far more powerful neighbor.
The telegram's revelation proved decisive. Six weeks later, on April 6, 1917, the United States declared war on Germany. What had been a European war became a World War in earnest, and the Zimmermann Telegram was the spark that lit it.