Who decoded the zimmermann telegram?

The Zimmermann Telegram was decoded by British intelligence, specifically by Room 40, a secret British naval intelligence unit. The telegram, sent by German Foreign Minister Arthur Zimmermann to Mexico in January 1917, proposed a military alliance between Germany and Mexico against the United States. Its interception and decryption played a crucial role in swaying public opinion in the U.S. towards entering World War I. The revelation of the telegram significantly impacted the course of the war.