This book examines the way in which the prospect of a wartime German assault on British seaborne commerce influenced the development of British naval policy in the run up to the First World War. It argues that, owing to the Admiralty’s consistently expressed fears that, in the event of an Anglo-German conflict, German commerce-raiders could interdict vital supplies, the British government began to consider German maritime power as a serious danger to British national security at the very outset of the twentieth century and that this sense of anxiety continued, even sharpened, as the years unfolded. It further argues that as a result of this perception of a growing menace, the Royal Navy devoted considerable time and energy to devising ever more elaborate countermeasures. These included developing new types of auxiliary and then regular warships, attempting to change international maritime law, creating a new global intelligence network, seeking to involve the government in the maritime insurance system and, finally, arming British merchant vessels and taking steps to place trained gun crews on these vessels in peacetime. While some of these developments have been subject to alternative explanations, some have never been explained at all. Yet, as this book shows, all had their origins, substantially or even entirely, in the Admiralty’s fears of a German threat to British maritime commerce. As a result, it concludes that the prospect of a German assault on British trade played a major part in shaping Admiralty policy in the twelve years before 1914.