The sinking of RMS Lusitania by a U-boat in May 1915 shocked Americans, forcing Germany to abandon the practice of unrestricted submarine warfare lest it draw the United States into World War I. In early 1917, however, wearily seeking speedy victory, the German high command decided to resume unrestricted submarine warfare.
At the same time, the Germans entertained an outlandish plot to encourage the Mexican conquest of the southern United States. As issued by German Foreign Secretary Arthur Zimmermann in an encrypted telegram, the plan also envisioned luring imperial Japan to conduct concurrent attacks against U.S. forces in Asia.
The British intercepted the “Zimmermann Telegram” and decrypted its text. By February 1917 the text had been shared with President Woodrow Wilson and the media. Facing few options, Wilson ordered Secretary of the Navy Josephus Daniels and Secretary of War Newton Baker to organize strategic plans for a potential U.S. declaration of war against imperial Germany.