On 7 May 1915, the German submarine U20 fired a torpedo at the passenger liner Lusitania. Eighteen minutes later the pride of the Cunard fleet disappeared beneath the waves. The sinking of the Lusitania ushered in a new, more savage era in naval warfare. It was a passenger vessel struck without warning by an unseen opponent; the victims were all civilians. Thus, both in its method and in its results, this action brought the stark brutality of 'total war' to the world's oceans.\n\nFor the Royal Navy, the sinking of the Lusitania has a further significance. The demise of this ship is proof for many of the backwardness of British naval thinking. That so important a vessel could be allowed to travel alone and unprotected in dangerous waters shows that no thought had been given by those in charge of Britain's maritime defences to the peril the country faced. Had the navy been truly prepared for 'total' warfare, so the argument runs, it would have anticipated that Germany would seek to defeat Britain with an attack on its trade and measures to protect British commerce would have been developed ahead of time and put into place from the outset.\n\nThis is a compelling argument, and it is true that Britain was not prepared for unrestricted submarine warfare. Yet, ironically, the Lusitania is proof that, before the First World War, the navy had given thought to the possibility of a German assault on British trade. For the liner that succumbed so dramatically to a German torpedo in 1915 had been conceived specifically to protect British commerce. The product of an agreement between Cunard and the government, the Lusitania was meant to serve as passenger vessels in peacetime but to become an auxiliary cruiser in wartime. To this end, it was built with turbines capable of generating a high speed, large coal bunkers designed to provide endurance, and pre-established fittings for gun mountings, intended to facilitate an easy-to-install offensive capability.\n\nThe Admiralty's decision to subsidize Cunard to build fast liners reflected the navy's belief that a new and dangerous threat to British commerce was being created. The threat in question came from Germany, whose fleet of Atlantic liners were viewed with apprehension. Intelligence suggested that these ships were capable of great speed, were manned largely by reservists and always had arms on board. Thus, the moment war broke out, it was feared that they would be converted into auxiliary warships and sent to prey on the trade routes. Because of their high speed no British merchantmen would be able to escape them and no British warships would be able to catch them. They would be in a position to run amok on the sea lanes; hence the need for British liners even faster to track them down.\n\nPaying Cunard to build the Lusitania was the first step in a twelve year history of efforts to counter the threat to British commerce from Germany's transatlantic liners. These efforts included radical new warship designs; a campaign to change international law to outlaw the conversion of liners on the high seas; and the establishment of a new global intelligence network to determine the location of German liners and route British ships away from them. Finally, in 1912 the decision was taken to arm British merchant vessels for their own defence.\n\nThese efforts to defend British trade from German attack absorbed considerable resources. Yet, despite the time and money devoted to this issue, the story of the threat from Germany's 'ocean greyhounds' and the British response has never been told. This project will remedy this. Focusing on the perceived threat posed by Germany, it will examine why the British naval authorities anticipated a danger from armed German liners and will explain how they chose to meet this challenge. This will illuminate an important but unknown area of our naval history and go some way to explaining why Britiain's trade defence policy was orientated in the wrong direction in 1914.