But this situation bore almost no resemblance to the struggle between the U-boats and the anti-submarine forces of the Allies. Barring a few naval actions between surface vessels, such as the battles of Jutland and of the Falkland Islands, the naval war was, for the most part, a succession of contests between single vessels or small groups of vessels. The enemy submarines sought to win the war by sinking the merchant shipping upon which depended the essential supplies of the allied populations and armies; and it was the effort of the Allies to prevent this, and to destroy submarines when possible, that constituted the vitally important naval activities of the war. By means of strategical and tactical dispositions, and various weapons and devices, now no longer secret, such as the depth charge, the mystery ship, hydrophones, mine-fields, explosive mine nets, special hunting submarines, and so forth, it was frequently possible either to destroy submarines with their entire crews, or to capture the few men who escaped when their boats were sunk, and thus keep from the German Admiralty all knowledge of the means by which their U-boats had met their fate. Thus the mystery ships, or decoy ships, as the Germans called them, destroyed a number of submarines before the enemy knew that such dangerous vessels existed. And even after they had acquired this knowledge, the mystery ships used various devices that enabled them to continue their successes until some unsuccessfully attacked submarine carried word of the new danger back to her home port. Under such unprecedented conditions of warfare, it is apparent that the Allied navies could not safely tell the public just what they were doing or how they were doing it. All articles written for the press had to be carefully censored, and all of these interesting matters ruthlessly suppressed; but now that the ban has been removed, it is desirable to give the relatives and friends of the fine chaps who did the good work sufficient information to enable them to understand the difficulty of the problem that was presented to the anti-submarine forces of the Allies, the manner in which it was solved, and the various means invented and employed. The subject is of course largely technical, but an effort has been made to present the story in such form that the layman can readily understand it. As it is difficult, if not quite impossible, for a naval officer to determine just which of the details that are a part of his daily life, and what incidents of sea experience would interest his civilian friends, the story has been written in collaboration with Mr. Burton J. Hendrick, to whom I am greatly indebted for invaluable assistance; and who, being an experienced hand at this writing business, deserves all the credit the reader may be disposed to accord him for both the form and such graces of descriptive style as he may be able to detect.