After the sinking of the Lusitania, a British passenger ship, by a German U-boat in 1915, the United States warned Germany that any further attacks on passenger ships would be considered an act of war. In 1917, Germany resumed unrestricted submarine warfare, sinking American merchant ships and killing American citizens. This led to the United States declaring war on Germany on April 6, 1917.
Zimmerman telegram
In January 1917, the British intercepted and decoded a telegram from German Foreign Secretary Arthur Zimmermann to the German ambassador in Mexico. The telegram proposed an alliance between Germany and Mexico in which Mexico would attack the United States and regain its lost territories in Texas, New Mexico, and Arizona. The telegram outraged the American public and helped to turn sentiment in favor of war with Germany.