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.: 429 761 people survived out of the 1,266 passengers and 696 crew aboard, and 123 of the casualties were American citizens. The sinking turned 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.