This second volume, dealing with the defeat of the enemy's submarine campaign, the gravest peril which ever threatened the population of this country, as well as of the whole Empire, may not be unwelcome as a statement of facts. They have been set down in order that the sequence and significance of events may be understood, and that the nation may appreciate the debt which it owes, in particular, to the seamen of the Royal Navy and the Mercantile Marine, who kept the seas during the unforgettable days of the intensive campaign. This book, therefore, gives the outline of the work accomplished by the Navy in combating the unrestricted submarine warfare instituted by the Central Powers in February, 1917. It would have been a labour of love to tell at greater length and in more detail how the menace was gradually overcome by the gallantry, endurance and strenuous work of those serving afloat in ships flying the White or the Red Ensigns, but I had not the necessary materials at my disposal for such an exhaustive record. The volume is consequently largely concerned with the successive steps taken at the Admiralty to deal with a situation which was always serious, and which at times assumed a very grave aspect. The ultimate result of all Naval warfare must naturally rest with those who are serving afloat, but it is only just to the Naval officers and others who did such fine work at the Admiralty in preparing for the sea effort, that their share in the Navy's final triumph should be known. The writing of this book appeared also to be the only way in which I could show my keen appreciation of the loyalty and devotion to duty of the Naval Staff, of the many clever, ingenious and audacious schemes developed and carried through for the destruction of submarines and the safeguarding of ocean-borne trade, and of the skilful organization which brought into being, and managed with such success, that great network of convoys by which the sea communications of the Allies were kept open. The volume shows how the officers who accompanied me to the Admiralty from the Grand Fleet at the end of 1916, in association with those already serving in Whitehall and others who joined in 1917, with the necessary and valuable assistance of our comrades of the Mercantile Marine, gradually produced the measures by which the Sea Service conquered the gravest danger which has ever faced the Empire.