In 1917, British code-breakers exposed a German plot against the United States which helped alter the course of World War One . The Americans had remained neutral during the first three years of war, but by 1917, Germany was planning to restart unrestricted submarine warfare which it feared would trigger America's entry into the war on the Allied side. German foreign minister, Arthur Zimmermann, proposed a Mexican attack on the United States.
Photo: (L) The Zimmermann telegram in code as sent from Washington to Mexico (R) A portion of the telegram as decrypted by British intelligence.(US National Archives and Record Administration)