In February, 1915, the German government announced an unrestricted warfare campaign. This meant that any ship taking goods to Allied countries was in danger of being attacked. This broke international agreements that stated commanders who suspected that a non-military vessel was carrying war materials, had to stop and search it, rather than do anything that would endanger the lives of the occupants.
The Lusitania, was at 32,000 tons, the largest passenger vessel on transatlantic service, left New York harbour for Liverpool on 1st May, 1915. It was 750ft long, weighed 32,500 tons and was capable of 26 knots. On this journey the ship carried 1,257 passengers and 650 crew.
There was some concern on board as a few days previously the German Embassy had published a statement that warned: "Travellers intending to embark for an Atlantic voyage are reminded that a state of war exists between Germany and her allies and Great Britain and her allies; that the zone of war includes the waters adjacent to the British Isles; that in accordance with the formal notice given by the Imperial German Government, vessels flying the flag of Great Britain or any of her allies are liable to destruction in those waters; and that travellers sailing in the war zone in ships of Great Britain or her allies do so at their own risk."
At 1.20pm on 7th May 1915, the U-20, only ten miles from the coast of Ireland, surfaced to recharge her batteries. Soon afterwards Captain Schwieger, the commander of the German U-Boat, observed the Lusitania in the distance. Schwieger gave the order to advance on the liner. The U20 had been at sea for seven days and had already sunk two liners and only had two torpedoes left. He fired the first one from a distance of 700 metres. Watching through his periscope it soon became clear that the Lusitania was going down and so he decided against using his second torpedo.
After a second, larger explosion, the Lusitania rolled over and sank in eighteen minutes. A total of 1,198 people died (785 passengers and 413 crew). Those killed included 128 US citizens.
The sinking of the Lusitania had a profound impact on public opinion in the United States. The German government apologized for the incident, but claimed its U-boat only fired one torpedo and the second explosion was a result of a secret cargo of heavy munitions on the ship. If this true, Britain was guilty of breaking the rules of warfare by using a civilian ship to carry ammunition. British authorities rejected this charge and claimed that the second explosion was caused by coal dust igniting in the ship's almost empty bunkers.