The United States Enters the War

Economic ties between the United States and the Allies were one underlying reasons for U.S. entry into the war in 1917. In addition, many Americans believed that the Allied nations were more democratic than the Central Powers were. A third reason was growing resentment against the Germans, especially for U-boat (submarine) attacks on ships carrying civilians, including Americans. On May 7, 1915, a German submarine attacked and sank the Lusitania, an ocean liner carrying more than 100 U.S. citizens among its passengers.

The event that finally pushed the United States into the war was the interception of the Zimmermann Telegram in January 1917. In this document, the German government offered to help Mexico reclaim territory it had lost to the United States in 1848 if Mexico allied itself with Germany in the war.