- German U-boat attacks on American ships: The sinking of the Lusitania, a British passenger liner, in May 1915, by a German U-boat, with the loss of 1,198 lives, including 128 Americans, was a major turning point in US public opinion. Although Americans were initially more outraged by British interference with American shipping, the Lusitania sinking made many Americans believe that Germany was a threat to American lives and interests.
- The Zimmerman Telegram: In January 1917, the British intercepted and decoded a telegram from German Foreign Secretary Arthur Zimmermann to the German ambassador in Mexico, proposing an alliance between Germany and Mexico in the event of war between the United States and Germany. The telegram promised that Germany would help Mexico recover "lost territory" in Texas, New Mexico, and Arizona. This revelation infuriated many Americans and further strengthened public support for entering the war against Germany.