On May 7, 1915, Imperial German Navy U-boat U-20 sank the RMS Lusitania off the coast of Ireland. The Germans attacked the Lusitania without warning and the ship went down within 18 minutes. 1198 people on board died. The passenger liner had departed from the port of New York City with many American citizens on board, 128 of whom were killed when the ship sank (including a member of the famous Vanderbilt family). The U.S. government condemned the German action and U.S. President Woodrow Wilson protested this violation of the United States' neutral rights and threatened to sever diplomatic relations with Germany.
In spite of this incident, Germany continued unrestricted submarine warfare until after the Arabic incident. On August 19, 1915, the British passenger liner Arabic was torpedoed by a German U-boat. Approximately 40 passengers and crew were lost, including two Americans. Wilson again vigorously protested this attack. With the threat that the United States might join the war, the German government issued on September 18, 1915, what became known as the "Arabic pledge," which stated that Germany would warn non-military ships 30 minutes before they sank them to make sure the passengers and crew got out safely.
Germany broke this pledge on March 24, 1916, when a U-boat torpedoed the French ship Sussex. This led to the Sussex pledge.