This PhD examines the role of British intelligence in Anglo-American relations during the period of American neutrality in the First \Vorld \Var. Unbeknownst to the Americans, British intelligence began to intercept and decrypt virtually all American diplomatic telegrams between Washington and U.S. diplomatic outposts throughout Europe. Although several studies of Anglo-American relations in this period exist, none consider British intelligence's role. Providing an analysis of the relevant cod.es and cryptographical developments during the war, the thesis traces British intelligence's progress in deciphering these various diplomatic codes and offers an analysis of the distribution and use of this intelligence material. Through an exploration of this intelligence aspect, this thesis challenges existing interpretations of British and American policy in this period. A crucial conflict at the heart of British policy-one missed by previous historians-existed over the importance of the United States. Presaging America's international role later in the twentieth centu1y, many of Britain's leaders came to seriously doubt that, without the United States, the war remained winnable at all. Yet these officials contended with a second, powerful faction that remained wedded to outmoded ideas of America's limited relevance on the global stage and that refused to accept the existence of practical limits to British power. This conflict play~ out in several areas of British policy-over diplomatic, military, financial, and political affairs. Intelligence, however, provea a favoured weapon. Intercepted communications, sometimes ripped from their context, caused serious but spurious paranoia that the Americans were collaborating with Germany. Previous scholars, however, by ignoring the weapon, have failed to see the battle. Until it entered the war, American policymakers worked t:u:elessly to achieve a peaceful settlement. Previous historians have entirely dismissed the significance of these efforts, casting them as well-intentioned but futile. In reality, however, those British leaders who understood Britain's dependence on the United States tended to favour these proposals as a useful way of ending an unwinnable war that was bleeding the country d17- This PhD makes a significant contribution to the history of British intelligence, British policy, and American diplomacy during the period of American neutrality during the First World War.