This book was written in the spring of 1943 with facilities supplied by the Navy Department and the Navy. Permission for its publication was then withheld by the Navy on grounds of national security. The permission to publish was granted two years later, in June 1945. Except for the appreciable deletions and other alterations made by the Navy censorship, the book is now published exactly as written. The reader will understand that where the present tense is used, as for example in the account of the German U-boats, the reference is to conditions existing in the spring of 1943. And if a commander today was a lieutenant then, he will accept our apologies for the use of his old title.