The Zimmermann Telegram was a secret diplomatic communication sent by the German Foreign Office to the German ambassador in Mexico. It proposed a military alliance between Germany and Mexico if the United States entered the war against Germany. In return, Germany promised to help Mexico recover its 'lost territory' in Texas, New Mexico, and Arizona. The message was intercepted and decrypted by British intelligence, who shared it with U.S. President Woodrow Wilson. Its publication in the American press caused national outrage and served as a major catalyst for the U.S. declaration of war against Germany in April 1917. This event effectively ended American isolationism and ensured that the vast industrial power and fresh manpower of the United States would be thrown behind the Allied cause.