Original and Decoded Zimmermann telegram, National Archives [source]
The Zimmerman Telegram
In 1917, WWI had been raging for two and half years, but the US had so far stayed out of it. But that was about to change. In January of 1917, the Germans sent a coded telegram through the US Embassy in Berlin to Mexico. The telegram contained a proposal to Mexico that if the US failed to remain neutral, Mexico would enter WWI on the side of the Central Powers and reconquer the “lost” territories of Texas, New Mexico, and Arizona. The British intercepted and decoded this transmission almost immediately, but it would take almost a month for them to come up with a plan to get the telegram to the US without revealing their spying on US telegram lines or their ability to crack the German codes. Along with the resumption of unrestricted submarine warfare, the Zimmerman Telegram is credited with being one of the chief reasons the US finally entered WWI.
For more information, check out this The Great War special on Mexico in WWI.