To challenge the British navy, Germany used their new weapon, the submarine. On May 1915, a German submarine sank a British ocean liner called Lusitania, killing 128 United States citizens from the 1200 passengers dead on board. This event was the first of many unreasonable acts by Germany. The following year the Sussex, a French passenger ferryboat, was sunk by Germany’s submarines. Wilson threatened Germany that the U.S. would break all diplomatic relations it had with Germany if the country continued sinking passenger ships. Initially Germany stopped all submarine warfare in agreement with the Sussex pledge which stopped Germany from harming any more passenger liners or merchant ships without the primary inspection of the carrier holding weapons. But on January 1917, Germany lifted all restrictions from the pledge. Germany resumed firing down any naval ships it viewed as a threat or determined as dangerous to its country. Wilson tried hard not to change his position on the war, but Germany really gave no other option. After Germany lifted its restrictions, they targeted the American cargo ship Housatonic, which a German U-boat sank on February 3,