Answer: They realized they had intercepted a message from the German foreign secretary.
Explanation: The excitement by the code breakers was present because it was Zimmermann's telegram, which was the secret diplomatic communication of the German Foreign Ministry in January 1917. In the telegram, the offer was sent to Mexico, by which Germany proposed a military alliance between Germany and Mexico in case the United States entered the First World War. In return, Mexico would get Texas, Arizona, and New Mexico.