Unrestricted  Submarine Warfare

There were unauthorized German submarines attacks during WWI. With this Germany sunk the Lusitania, a British transport ship, bound for Britain from New York. The German U-boats wanted to sink all supply ships going to Britain in order to starve them. 1195 people died including 128 Americans. The Lusitania's sinking was the biggest influence on the American decision to enter the war. German submarine warfare sank many ships of many countries during the war.

                           Zimmerman Telegram

The Zimmerman Telegram finally pushed the US to war. The Zimmerman Telegram was sent from the German  to Mexico. It proposes an alliance with Mexico and said If the US goes to war, Mexico must fight and financially support  Germany. Also If Mexico agrees to fight, they will reconquer New Mexico, Texas and Arizona.
The telegraph was intercepted by British Intelligence and sent to the American government by the Brits. This made Americans so angry.