March 1, 1917
Two moves by the Germans brought the United States into World War I: Germany’s decision to wage unrestricted submarine warfare and the Zimmermann Telegram. On March 1, 1917, the United States intercepted a telegram from Arthur Zimmermann, German foreign secretary, to Mexico. The coded correspondence proposed an alliance between Mexico and Germany, and vaguely promised Mexico the return of Texas, New Mexico, and Arizona if Mexico helped Germany attack the United States. This and the new German policy of unrestricted submarine warfare forced the United States to forego neutrality and enter World War I on April 2, 1917. The entrance of America on the side of Great Britain and France gave the Allies the last push forward to win the war.