Had Lusitania survived the Great War, she and her equally celebrated sister Mauretania might well have become the most successful pair of liners ever to ply the North Atlantic trade. As it was, she was destined to achieve a quite different sort of immortality as the first large passenger ship in the long history of sea warfare to be sunk by a submarine without warning in an underwater attack. Apart from the brutality of her sinking, her loss also hastened - if not precipitated - the United States' entry into the global conflict which, once that occurred, guaranteed Germany's defeat. Had Kapitänleutnant Schwieger, the submarine's commander, paused to consider this eventuality, perhaps he might have left the world's fastest steamer unmolested and thereby allowed history to take a different course.

Lusitania and Mauretania were conceived as the British response to J. Pierpont Morgan's acquisition of the White Star Line, a move which not only threatened Cunard's domination of the transatlantic ferry but also wounded national pride. Designed on the grand scale, the pair had to be financed from government loans in return for a promise that both ships could be used as auxiliary cruisers in time of war. In view of their size, the orders for the two vessels went to separate yards, with that for Lusitania going to John Brown at Clydebank where she was launched just ahead of her sister on 7th June 1906. Luxuriously appointed, with accommodation for 563 First, 464 Second and 1,138 Steerage passengers in the care of 802 crew, she was, when delivered on 26th August 1907, the largest ship in the world. Registered at 31,550 tons gross, she measured 787 feet in length with an 87½ foot beam, and could cruise at 25 knots driven by quadruple screws. Clearing Liverpool on 7th September for her maiden voyage to New York, she siezed the 'Blue Riband' the following month when she set record times for both outward and return crossings at average speeds of 23.99 and 23.61 knots respectively. Even though she subsequently surrendered these records to Mauretania, the courageous decision to fit both sisters with turbines rather than conventional reciprocating engines had proved a triumph and in the years up to 1914, no ship afloat could match the two Cunarders for speed.

When war was declared in August 1914, the authorities immediately realised that the twin sisters were actually too large for conversion to armed merchant cruisers but whereas Mauretania was laid up pending duty as a troop transport, Lusitania continued her regular Liverpool to New York service as if immune to the international situation. Well patronised by passengers who perceived her speed as their best safeguard against enemy attack, her Atlantic crossings soon became almost routine and there was an element of complacency about the risks involved. On 4th February 1915, the German Government had declared the waters around Great Britain and Ireland to be a war-zone within which all enemy shipping was liable to be sunk. On 1st May, the German Embassy in Washington published a reminder of this declaration in the New York papers alongside Cunard's sailing schedules. Such was the belief in Lusitania's invulnerability however, only a handful of passengers cancelled their booking for that day's departure and the liner left Pier 54 at 10.30am. with 189 Americans included amongst the 1,959 people aboard. The voyage from New York was uneventful and apart from being slower than usual due to having six boilers shut down to conserve coal, Lusitania rounded the Fastnet Rock on the evening of 6th May. Her master Captain Turner had already received warnings of submarine activity in the Irish Sea and had reduced speed further so as to approach the area in darkness. The next morning, Friday 7th May, was calm but foggy, ideal U-boat weather, and Turner was doubly glad that the cruiser H.M.S. Juno would soon be arriving from Queenstown to escort him to Liverpool. As the day wore on with no sign of Juno making her rendezvous, Turner altered course to take Lusitania closer inshore; it was a fatal decision and one which was to haunt Captain Turner for the rest of his life.

Unbeknown to Turner, his new course lay directly across that of the German submarine U-20 which, under the command of Kapitänleutnant Walter Schweiger, was patrolling the Irish Sea under orders to sink any British vessels he encountered. His lookouts spotted Lusitania just after 1 o'clock and, submerging immediately, he shadowed the liner for about an hour even though he was unsure whether it was Lusitania or her sister Mauretania. For him, the precise identification of the quarry was almost irrelevant since both were classed as auxiliary cruisers and to sink either would bring him the greatest accolade of his career. He carefully manoeuvered into position and at 2pm. fired a single G-type torpedo from one of the U-20's forward tubes. It hit Lusitania on her starboard side just aft of the bridge and within moments, the initial explosion was followed by another that was much louder and more violent. In an instant the great ship lost way and began heeling over. Even though the lifeboats were already swung out as a precaution, it proved impossible to launch those on the port side due to the sharp list and there was panic and confusion on the boatdeck. Despite her profusion of watertight compartments, the liner flooded and began to settle astonishingly quickly. Even Schwieger was amazed and just eighteen minutes after the torpedo had struck, Lusitania slipped beneath the waves taking almost 1,200 souls down with her.

Although a flotilla of small local craft rescued 764 survivors, the loss of the unarmed passenger ship along with almost two-thirds of those aboard her provoked bitter condemnation of Germany and her policy of unrestricted submarine warfare. The neutral nations, most especially the United States where the public was stunned by the deaths of 134 of their fellow citizens, reacted with disgust and indignation which Germany attempted to diffuse by alleging that Lusitania had been loaded with munitions, the explosion of which had caused her to founder so quickly. The riddle of her rapid sinking has remained controversial almost to the present day although Dr. Robert Ballard believes he has now found the answer. Ballard, fresh from his successes in locating the wrecks of Titanic and Bismarck, has also dived on the wreck of Lusitania and concludes that U-20's single torpedo ignited methane gas which had built up in the near-empty coal bunkers to cause a massive explosion which blew out large sections of the ship's side. Whatever the truth of this claim, the sinking of the Lusitania shocked the civilised world more profoundly than almost any other single event of the War and it remains to this day one of the greatest maritime tragedies of all time.