Alfred Zimmermann's Telegram
On January 19, 1917, Alfred Zimmermann, German Foreign Secretary, sent a telegram to the Mexican government asking Mexico to invade America. The thinking behind this was that if the United States was occupied with defending itself against Mexico at home, then it would not be able to get involved in the fight in Europe. In exchange for invading America, Germany promised Mexico the states of Texas, New Mexico, and Arizona.
Alfred Ewing (a British cryptographer) and his "Room 40" team of code breakers, intercepted the telegram, decoded it, and sent it to England's director of navel intelligence, Sir William Hall. On February 5, 1917, Hall intercepted another message from Germany to Mexico asking the Mexican government to move ahead with the plan without waiting for America to declare war. The British then sent the details of this message to President Woodrow Wilson. With Woodrow Wilson's permission, the telegram was published on March 1, 1917. The American people were outraged. America was getting closer to entering the war.