My former high school history teacher is an awesome guy. He's basically a real life Forrest Gump. Anyways, one of the things he taught my class was that he thought the Zimmerman Telegraph to was fake. His main evidence for this was the German's refuting that they sent it, and that it was sent along British telegraph lines. Is this just a conspiracy theory? Or does it have some merit and backing as a theory by credible historians?
Arthur Zimmermann, the German who wrote the telegram himself acknowledged the authenticity of the telegram repeatedly in 1917. Stressing the fact that it was only suppose to be in the context of american entrance into the war that Germany would support mexico. There was widespread belief that it was a hoax among the general public before that, but Zimmermann's admittance removed such doubt's.
While u/DeSoulis has addressed the German acknowledgement of the telegram's authenticity, I'd like to point out that the fact that it was sent along British telegraph lines didn't increase the likelihood of falsity. On August 5th 1914, the cable ship Telconia cut undersea telegraph cables in the North Sea. These ran from Germany to Western Europe, North Africa and the USA. The only cables left untouched were those that ran to the UK. As a result, all German telegraph messages sent during the war had to pass through the British system, including official traffic. The Zimmerman Telegraph being sent through British cables would have drawn little comment, because it could only have been sent through those cables.