During the First World War German use of unrestricted submarine warfare, supported by extensive mining and surface raids, very nearly forced Britain out of the war in 1917. The island's heavy dependence on seaborne supplies was gravely threatened again in 1939, supplemented this time by air attacks on shipping. After the war Commanders Waters and Barley wrote a Naval Staff History which has long been recognised as an authoritative study of the impact of the German campaign and its ultimate defeat by Britain and her allies. It remains an indispensable basis for any serious study of the Battle of the Atlantic and has here been updated and revised by Dr Grove, who also contributes a perceptive introduction outlining its significance.