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How did British codebreakers help bring the United States into World War I?

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George W. Bush and Porter J. Goss at CIA headquarters in Langley, Virginia. — intelligence U.S. Pres. George W. Bush (front) and Porter J. Goss, director (2004–06) of the Central Intelligence Agency (CIA), heading to a press conference at CIA headquarters in Langley, Virginia, March 3, 2005.

British codebreakers played a pivotal role in shifting American public opinion toward entering World War I by intercepting and deciphering the Zimmermann Telegram.12

In January 1917, German Foreign Secretary Arthur Zimmermann sent a coded message to the German ambassador in Mexico, proposing a military alliance against the United States. The proposal promised that if the U.S. entered the war, Mexico could recover its lost territories of Texas, New Mexico, and Arizona.13

British Admiralty Intelligence intercepted and decoded this message, communicating it to President Woodrow Wilson in February 1917. When the telegram was published in the U.S. press on March 1, it served as definitive proof of German hostility toward the United States. This revelation, combined with ongoing German submarine attacks on merchant ships, galvanized American public sentiment and became a critical factor in the U.S. declaration of war against Germany on April 6, 1917.1324