Anglo-Irish relations in the twentieth century can be described as being close but tortuous. This paradox is fused with Ireland's geographical location - both isolated from Europe and in close proximity to the main island of the British archipelago. Using a geopolitical analysis based on the theories of Sir Halford Mackinder, this book provides a new understanding of the strategic imperatives that have driven British policy throughout the turbulent events of the twentieth century. Containing material which has only recently been released by the Public Record Office, this book brings an entirely new perspective to the reality of Irish neutrality, and the pivotal importance of Northern Ireland in the Battle of the Atlantic during the Second World War. Furthermore, using US archival material, it gives a new insight into Ireland's geopolitical importance in the First World War, and her contribution to victory against the German U-boats.