In February 1917 U.S. Pres. Woodrow Wilson was made aware of the Zimmermann Telegram, a coded message sent by German foreign secretary Arthur Zimmermann. The telegram proposed that Mexico enter into an alliance with Germany against the United States, promising Mexico the return of its “lost provinces” of Texas, Arizona, and New Mexico. The publication of the telegram caused an uproar, and American opinion began to swing in favor of entering the war against Germany. At the same time, Germany resumed its practice of unrestricted submarine warfare and German U-boats began sinking American merchant ships in March. On April 2, 1917, Wilson addressed a joint session of Congress, declaring that “The world must be made safe for democracy.” The U.S. Congress declared war on Germany on April 6.