The RMS Lusitania was a UK-registered ocean liner that was torpedoed by an Imperial German Navy U-boat during the First World War on 7 May 1915, about 11 nautical miles (20 kilometres) off the Old Head of Kinsale, Ireland. The attack took place in the declared maritime war-zone around the UK, shortly after unrestricted submarine warfare against the ships of the United Kingdom had been announced by Germany following the Allied powers' implementation of a naval blockade against it and the other Central Powers. The passengers had been warned before departing New York of the danger of voyaging into the area in a British ship.
The Cunard liner was attacked by U-20 commanded by Kapitänleutnant Walther Schwieger. After the single torpedo struck, a second explosion occurred inside the ship, which then sank in only 18 minutes. The U-20’s mission was to sink and destroy British warships and liners in the area between the North Sea and the Irish Sea. 761 people survived out of 1962 (1,266 passengers and 696 crew) aboard, with American citizens making up 123 of the casualties. The sinking turned the public opinion in many countries against Germany. It also contributed to the American entry into the War two years later; images of the stricken liner were used heavily in US propaganda and military recruiting campaigns.
The contemporary investigations in both the United Kingdom and the United States into the precise causes of the ship's loss were obstructed by the needs of wartime secrecy and a propaganda campaign to ensure all blame fell upon Germany. Argument over whether the ship was a legitimate military target raged back and forth throughout the war, but after the war it was revealed that at the time of her sinking she was carrying over 4 million rounds of machine-gun ammunition of .303 calibre, almost 5,000 shrapnel shell casings (for a total of some 50 tons), and 3,240 brass percussion artillery fuses.
Several attempts have been made over the years since the sinking to dive to the wreck seeking information about how the ship sank, and military ammunition has been discovered in the wreck. The argument continues to the present day.