On 7 May 1915, the famed Cunard liner Lusitania was struck by a torpedo from a German U-boat off the coast of Ireland, with more than 1000 lives lost.

The Lusitania, which had a capacity for 3000 passengers, had left New York a week before, bound for Liverpool, England. She was unarmed but had been carrying a cargo of rifle ammunition and shells. Because of German submarine presence in the Irish coastal waters, the British Admiralty had advised ships to steer a zigzag course as an evasive tactic, but the Lusitania appeared to have been travelling in a straight course.

Lusitania was suddenly attacked at 2.15pm and was struck by a single torpedo which blew a hole in her side. A second bigger explosion followed, either from a second torpedo or an explosion of the ship's boilers. The 32,000 tonne liner went down in 94 metres of water within 20 minutes, tilted so steeply that it was many lifeboats could not be launched.

Of the 1959 passengers and crew aboard, 1198 were lost, including 124 of the 218 Americnas, among them the millionaire Alfred Gwynne Vanderbilt.

Germany justified the action by claiming that the ship was armed and carrying stolen ammunition. However both Allied and neutral countries condemned their actions. US President Woodrow Wilson, although maintaining his country's neutrality, reacted strongly against the outrage; ex-President Theodore Roosevelt called it "piracy on a vaster scale than the worst pirates of history".

The sinking of the Lusitania nearly caused the USA to enter World War I. However, it was only on 6 April 1917, that the USA no longer remained neutrality and announced its entry into the war, to "make the world safe for democracy".