Britain
Britain secretly got the Zimmerman telegram and was able to decode it.
The History of the Zimmerman Telegram
The Zimmerman Telegram was a telegram made by Germany. Britain was making Germany angry, so they decided they weren't going to keep their promise to slow down the submarine attacks on February the 1st, 1917. Germany sent it to Mexico saying that they hope that the United States of America remains neutral.
If it doesn't work out, they're promising Mexico all of these amazing things if they form an alliance, including getting some states that America took from them. Weeks after, Britain found this and presented it to U.S.A. to get them to hate Germany, too. It worked, and the United States published this and declared war on Germany and all of the countries they had an alliance with.
The reason this document is so important is because it was what got the United States to join World War I and it was used as a weapon for Germany from Germany itself.
If you would like to learn more about the Zimmerman Telegram, I would suggest visitng the National Archives and the National World War I Museum and Memorial Website.
The Life of A Secret Agent
Hi, I'm the Zimmerman Telegram. I don't know if this is okay to say, but I'm a secret agent. I was born on January 1917 in Germany. You're probably laughing, saying, "How in the world can a piece of paper be a secret agent?" Well, here's how. I was written by a man named Arthur Zimmerman, but not normally. I was written in a secret code, so no one could know whatever Secretary Zimmerman wrote. He wrote things saying that Germany was not actually going to be keeping the promise of stopping submarine
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attacks on February 1st, 1917. They were actually going to START a submarine attack on February 1st, 1917! They wanted to keep U.S. neutral, but if they didn't stay neutral, Germany promised Mexico amazing things things if they form an alliance with them, including getting some states the United States took from them! I was very happy, feeling super important, with me almost being like a rule breaker. But then Britain found me. How they did? I don't really remember, I just remember a bunch of these British guys looking down one me, speaking about what to do with me. I heard them talking about how this was the perfect oppurtunity to get the United States of America into this