German submarines, known as U-boats, sank hundreds of Allied ships, including passenger ships, without warning. The most famous incident was the sinking of the British passenger liner Lusitania in 1915, which killed over 1,200 people, including 128 Americans.
Zimmerman Telegram
The Zimmerman Telegram was a secret message sent by the German foreign secretary, Arthur Zimmermann, to the German ambassador in Mexico, Heinrich von Eckardt. The message proposed an alliance between Germany and Mexico in the event of war between the United States and Germany. The telegram was intercepted and decoded by British intelligence and released to the press in the United States. The publication of the telegram caused outrage in the United States and helped turn public opinion in favor of war against Germany.