How did British codebreakers use the Zimmermann Telegram during World War I?

During World War I, British Admiralty intelligence successfully intercepted and decoded the Zimmermann Telegram, a secret communication from German Foreign Secretary Arthur Zimmermann to the German minister in Mexico.12

The telegram proposed a military alliance between Germany and Mexico, promising German support for Mexico to reconquer territory in Texas, New Mexico, and Arizona if the United States entered the war against Germany. Recognizing the strategic value of this intelligence, the British shared the decoded message with the U.S. government.13

President Woodrow Wilson subsequently authorized the publication of the telegram on March 1, 1917. By revealing Germany's hostile intentions toward the United States, the disclosure played a pivotal role in shifting American public sentiment and served as a significant factor in the U.S. decision to declare war on Germany five weeks later. To protect their cryptanalysis capabilities, the British role in intercepting and deciphering the message was not publicly disclosed at the time.12