The German Empire used U-boats (Unterseeboot) to attack enemy ships and create blockades against supply ships. These submarines worried the British who depended on food imports to keep the war effort going, because they could easily sink the ships carrying the rations.
U-boats could carry 35 men and could travel underwater for 2 hours. It was armed with 12 torpedoes. On May 7, 1915 the Lusitania was attacked. Many on board who were killed were neutral Americans. The German Empire which had a sink on sight policy for any shipping coming towards Britain had to apologise to the United States who were at the time not part of the war.
Germany argued that while a civilian liner, the Lusitania was carrying contraband munitions on board which made it a threat to the war effort.
In early 1917 Germany resumed its U-boat attacks on any ship coming towards Britain. This was known as ‘unrestricted submarine warfare’. Germany as a result, sank the American liner Housatonic and several American merchant ships prompting the nation to enter into the war.