The Sussex was a passenger ferry that was used by people to cross from France to England. During World War I, the ship was torpedoed by a German submarine. The German policy at that point was to torpedo any merchant ships, but not passenger ships, without warning. The ship did not sink, but many passengers were killed. After the attack, President Wilson threatened to break diplomatic relations with Germany if this sort of unrestricted submarine warfare did not stop. Germany then issued the "Sussex Pledge" in which it promised to make changes in its policy regarding submarine warfare. These changes held until early 1917, when Germany resumed unrestricted submarine warfare, helping to cause the US to enter the war.