World War I began in 1914 as a fight among several European nations, while the United States pursued a policy of non-intervention. In fact, Woodrow Wilson was reelected President largely because “He kept us out of war”. But as the war unfolded in Europe, an intercepted telegram sent by the German Foreign Secretary, Arthur Zimmermann, to the Mexican government inflamed the U.S. public opinion and was one of the main reasons for the entry of the U.S. into WWI. This is the story of the encrypted telegram that changed the last century.